Eukaryotic Gene Regulation PDF

Concepts of Genetics Twelfth Edition Chapter 17 Transcriptional Regulation in Eukaryotes Lecture Presentation by...

Concepts of Genetics Twelfth Edition Chapter 17 Transcriptional Regulation in Eukaryotes Lecture Presentation by Dr. Cindy Malone California State University Northridge Copyright © 2019 Pearson Education, Inc. All Rights Reserved Eukaryotic Gene Regulation Eukaryotic gene regulation is more complex than that in prokaryotes Copyright © 2019 Pearson Education, Inc. All Rights Reserved Eukaryotic Gene Regulation Eukaryotic gene regulation is more complex than that in prokaryotes – Larger genomes, with more non-coding DNA – DNA is associated with histones and other proteins – Genes are enclosed in a double membrane bound nucleus – mRNAs must be spliced, capped, and polyadenylated prior to transport from nucleus – Some eukaryotes are multicellular and require complex gene expression patterns Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Differential regulation of gene expression in multi-cellular eukaryotes Multicellular organisms must regulate gene expression in different cells, tissues, organs - This involves spatial regulation – WHERE a gene or set of genes is expressed (tissue, cell type, etc.) - This also involves temporal regulation of gene expression WHEN a gene or set of genes is expressed (All the time? Just in response to a specific signal?) In other words, different cells express different genes, and gene expression within a cell can change over time In multi-cellular eukaryotic organisms, somatic cells in different tissues share the same genetic instructions (DNA) but have very different characteristic forms and functions. How is this possible? zygote embryo adult In multi-cellular eukaryotic organisms, somatic cells in different tissues share the same genetic instructions (DNA) but have very different characteristic forms and functions. How is this possible? Differential utilization of genome by cells via differential regulation of gene expression! zygote embryo adult Each cell expresses a distinct subset of genes in the genome  Some genes (“housekeeping genes”) like actin, a component of the cytoskeleton, are regularly expressed in most cells Other genes are “lineage specific” and are expressed only in certain cell types. These genes can help cells differentiate and develop different characteristics Expression of some genes is induced in response to a signal – they are not expressed all the time. Molecular Cell 2018 71389-397DOI: (10.1016/j.molcel.2018.07.017) So how do eukaryotes regulate when and where a gene is expressed? Gene expression is complicated, and regulation can take place during different parts of the process. Each cell type reads a unique combination of genes Both the genome and epigenome are important for interpreting the instruction manual DNA sequence chromatin Eukaryotic Gene Regulation Do the in-class activity to refresh your memory of ways in which eukaryotic gene expression can be regulated. Copyright © 2019 Pearson Education, Inc. All Rights Reserved Figure 3.5 Steps in the production of β-globin and hemoglobin  Regulation of gene expression in eukaryotes In eukaryotes, production and activity of gene products (proteins or RNAs) can be regulated at many levels – Regulation of transcription – Post-transcriptional mechanisms Modification of RNA - Degradation of RNA, RNA interference - Alternative splicing of RNA primary transcript Regulation of translation initiation Post-translational mechanisms (modifications made to protein product…) Regulation of gene expression at the level of transcription 2 major ways to do this play important roles in regulating gene expression during development: 1) Regulation of chromatin structure in the area where the gene and/or regulatory sequences are located 2) Use of transcription factor proteins (general and specific). (Note that these often help regulate chromatin structure) Regulation of gene expression at the level of transcription 2 major ways to do this play important roles in regulating gene expression during development: 1) Regulation of chromatin structure in the area where the gene and/or regulatory sequences are located 2) Use of transcription factor proteins (general and specific)  Eukaryotic Chromosome Structure refresher Eukaryotic DNA is packaged into chromatin. Chromatin is comprised of which macromolecules? DNA and associated proteins Changing chromatin structure (how closely or loosely the DNA is packaged) can alter gene expression by making it easier or harder for the proteins involved in gene expression to access the gene http://csma31.csm.jmu.edu/chemistry/ faculty/mohler/research/IMAGE7.GIF Euchromatin and Heterochromatin Euchromatin – ‘Open’ chromatin, uncoiled/loose chromatin structure – Associated with active gene expression Heterochromatin – ‘Closed’ chromatin, condensed chromatin structure – Generally inactive regions of the genome, little to no gene expression Copyright © 2019 Pearson Education, Inc. All Rights Reserved Figure 3.4 Nucleosome and chromatin structure http://kc.njnu.edu.cn/swxbx/shuangyu/8.htm Gene Expression Influenced by Chromatin Modifications Chromatin modifications may include – Changes to histones ▪ Histone modifications (methylation, acetylation, phosphorylation) ▪ Histone variants (alternative histone proteins) ▪ Change in histone placement – DNA methylation – Binding of other proteins to DNA All of these things can influence chromatin packaging and structure Copyright © 2019 Pearson Education, Inc. All Rights Reserved Histone Modification Modifications occur at conserved amino acid sequences in the N-terminal histone tails, which protrude from the nucleosome – Most common additions are acetyl, methyl, or phosphate. Chemical modification of histones alters the structure of chromatin, making genes accessible or inaccessible for transcription Copyright © 2019 Pearson Education, Inc. All Rights Reserved Chromatin can be “remodeled” (condensed or opened) via methylation or acetylation, and changes in chromatin structure can affect transcription Acetylation of histones can result in a more open local chromatin conformation, making it easier for transcription factors and RNA Polymerase to access the DNA and leading to increased transcriptional activation Histones and chromatin conformation Nucleosome - chromosomal DNA wrapped around a histone octamer 2x(H2A, H2B, H3, H4). H1 on linker DNA region separating nucleosomes. Acetylation occurs on the “tails” of histone proteins that extend from globular core in the center of the nucleosome.  Histone acetylation – why does it affect chromatin structure? Acetylation of basic Lysine residues on a Histone H4 negates a positive charge. This decreases the attraction to the negatively- charged DNA, allowing the chromatin to adopt a more open conformation. neutral charge positive charge HAT = histone acetyl transferase HD = histone deacetylase Histone Modification Histone Acetylation Acetylation by histone acetyltransferase (HAT) opens up the chromatin structure – Makes genes available for transcription Removal of the acetyl groups by histone deacetylase (H DAC) closes the configuration – Silences genes by making them unavailable Copyright © 2019 Pearson Education, Inc. All Rights Reserved Histone Modification Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Methylation of DNA (for example, at CpG islands) can result in condensed (“closed”) conformation of chromatin, leading to transcriptional repression (less gene expression) DNA Methylation One of the major epigenetic mechanisms is the reversible modification of DNA by the addition or removal of methyl groups Copyright © 2019 Pearson Education, Inc. All Rights Reserved DNA Methylation Methylation involves the addition of a methyl group catalyzed by methyltransferase enzymes Addition of a methyl group (-CH3) to cytosine on the 5-carbon of the cytosine nitrogenous base resulting in 5-methylcytosine (5mC) Reaction catalyzed by a family of enzymes called DNA methyltransferases (DNMTs) Copyright © 2019 Pearson Education, Inc. All Rights Reserved DNA Methylation Methylation occurs on cytosine bases adjacent to guanine called CpG dinucleotides, which are clustered in regions called CpG islands CpG islands are located in and near promoter sequences adjacent to genes – CpG Islands adjacent to essential genes (housekeeping genes) and cell-specific genes are unmethylated and available for transcription – Other genes are methylated and transcriptionally silenced Copyright © 2019 Pearson Education, Inc. All Rights Reserved DNA Methylation Copyright © 2019 Pearson Education, Inc. All Rights Reserved DNA Methylation The added methyl groups occupy the major groove of DNA and silence genes by blocking the binding of transcription factors and other proteins necessary to form transcription complexes Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Other Types of Histone Modification SWI/SNF – additional complex of proteins - remodels chromatin – Loosens attachment between histone and DNA – Loosens DNA strand from nucleosome core – Causes reorganization of internal nucleosome component Copyright © 2019 Pearson Education, Inc. All Rights Reserved  lncRNA (Intergenic noncoding RNA) Unlike mRNA, lncRNAs lack a frame that codes for the insertion of amino acids into a polypeptide lncRNA binds to chromatin-modifying enzymes – Affects chromatin modification and gene expression Copyright © 2019 Pearson Education, Inc. All Rights Reserved Introduction to Epigenetics An epigenetic trait is a stable, mitotically and meiotically heritable phenotype that results from changes in gene expression without alterations in the DNA sequence Epigenetics is the study of the ways in which these changes alter cell- and tissue-specific patterns of gene expression The epigenome refers to the epigenetic state of a cell These epigenetic changes are not mutations (alterations of the DNA sequence) but can be inherited. Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Epigenetic Alterations to the Genome Epigenetic changes occur through three major mechanisms: – DNA methylation – Histone modification – Noncoding RNA Copyright © 2019 Pearson Education, Inc. All Rights Reserved Introduction to Epigenetics – Epigenetic changes lead to phenotypic changes throughout the organism's life cycle. Copyright © 2019 Pearson Education, Inc. All Rights Reserved Environmental Induction of Epigenetic Change Environmental agents including nutrition, chemicals, and physical factors such as temperature can alter gene expression by affecting the epigenetic state of the genome The clearest evidence for the role of environmental factors comes from studies in experimental animals Copyright © 2019 Pearson Education, Inc. All Rights Reserved Copyright © 2019 Pearson Education, Inc. All Rights Reserved Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Environmental Induction of Epigenetic Change This change in coat color was NOT caused by changes in the DNA sequence This coat color change was due to changes in DNA methylation patterns near a particular gene Epigenetic inheritance Copyright © 2019 Pearson Education, Inc. All Rights Reserved Introduction to Epigenetics Epigenetics has been implicated in: – Progressive restriction of gene expression during development – Allele-specific expression in gene imprinting – X-chromosome inactivation – Environment genome interactions during prenatal development that affect adult phenotypes – The loss or alteration of other epigenetic states can result in cancer or other disorders Copyright © 2019 Pearson Education, Inc. All Rights Reserved Epigenetics and Imprinting Imprinted genes show expression of only the maternal allele or the paternal allele Differential methylation of CpG-rich regions produces allele-specific imprinting and subsequent gene silencing Copyright © 2019 Pearson Education, Inc. All Rights Reserved Epigenetics and Imprinting Once a gene has been methylated and imprinted, it remains transcriptionally silent during embryogenesis and development Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Epigenetics and X Chromosome Inactivation X chromosomes in mammalian females are inactivated through epigenetic changes converting them into heterochromatin (dosage compensation) Copyright © 2019 Pearson Education, Inc. All Rights Reserved Barr Bodies Barr bodies – The inactivated X chromosome condenses to form a Barr body – A Barr body is highly condensed heterochromatin that prevents the expression of genes XX XY Barr body Copyright © 2019 Pearson Education, Inc. All Rights Reserved Epigenetics and Cancer Cancer involves both epigenetic and genetic changes that lead to alterations in gene expression Copyright © 2019 Pearson Education, Inc. All Rights Reserved Copyright © 2019 Pearson Education, Inc. All Rights Reserved Section 19.3 Epigenetics and Cancer Several lines of evidence support the role of epigenetic alterations in cancer: 1. Global hypomethylation may cause genomic instability and the large-scale chromosomal changes that are a characteristic feature of cancer 2. Epigenetic mechanisms can replace mutations as a way of silencing individual tumor-suppressor genes or activating oncogenes 3. Epigenetic modifications can silence multiple genes, making them more effective in transforming normal cells into malignant cells than sequential mutations of single genes Copyright © 2019 Pearson Education, Inc. All Rights Reserved Twin studies have also shed light on epigenetic change and inheritance. Identical twins do not always get the same diseases or share all the same traits. They share many of the same genetic markers (gene expression) when they are younger, but these markers begin to differ more and more over time. http://learn.genetics.utah.edu/content/epigenetics/  Regulation of gene expression in eukaryotes In eukaryotes, production and activity of gene products (proteins or RNAs) can be regulated at many levels – Regulation of transcription – Post-transcriptional mechanisms Modification of RNA - Degradation of RNA, RNA interference - Alternative splicing of RNA primary transcript Regulation of translation initiation Post-translational mechanisms (modifications made to protein product…) Regulation of gene expression at the level of transcription 2 major ways to do this play important roles in regulating gene expression during development: 1) Regulation of chromatin structure in the area where the gene and/or regulatory sequences are located 2) Use of transcription factor proteins (general and specific). (Note that these often help regulate chromatin structure)  Transcription factors are proteins that bind to DNA and regulate (you guessed it) transcription – general transcription factors (also known as basal transcription factors) are required for transcription initiation in all cells. They must be present in order for RNA polymerase to associate strongly with the promoter and begin transcription – specific transcription factors regulate transcription in certain cells or in response to signals. Such factors are often tissue-specific and may only be expressed themselves at certain times, resulting in spatial and temporal regulation of expression of downstream genes. These specific transcription factors are the ones that allow for differentiation of cells  Cis-Acting Sequences Cis-acting sequences – Located on same chromosome as gene that it regulates – Required for accurate regulated transcription of genes ▪ Promoters ▪ Enhancers ▪ Silencers Copyright © 2019 Pearson Education, Inc. All Rights Reserved Promoters Promoters – Nucleotide sequences that serve as recognition sites for transcription machinery – Located immediately adjacent to regulatory genes – Critical for transcription initiation – Core promoter: Determines accurate initiation of transcription – Proximal-promoter elements: Modulate efficiency of basal levels of transcription Copyright © 2019 Pearson Education, Inc. All Rights Reserved The Initiation of Transcription at a Eukaryotic Promoter 1) A eukaryotic promoter 2) Several transcription factors bind to DNA. 3) Transcription initiation complex forms. Copyright © 2019 Pearson Education, Inc. All Rights Reserved © 2017 Pearson Education, Inc.  Enhancers and Silencers Enhancers and silencers – regulatory sequences where specific transcription factors (TF) bind – Found upstream, within (introns), or downstream of gene – Binding of TF to enhancers increases transcription levels; binding to silencers (repressors) decreases them – Modulate transcription from a distance – Act to increase or decrease transcription in response to cell’s requirement for gene product. These regions are found in all cells but each one may be used in some cells but not others Copyright © 2019 Pearson Education, Inc. All Rights Reserved Figure 3.6 The bridge between enhancer and promoter can be made by transcription factors  DNA in both cells Enhancer for contains the albumin albumin gene Promoter Albumin gene gene and the crystallin gene: Control Enhancer for elements crystallin gene Promoter Crystallin gene LIVER CELL NUCLEUS LENS CELL NUCLEUS Available Available activators activators Albumin gene Albumin not expressed gene expressed Crystallin gene not expressed Crystallin gene expressed (a) Liver cell (b) Lens cell © 2017 Pearson Education, Inc. Cambell Biology 11th ed Figure 3.7 Enhancer region modularity Enhancers are often used to drive tissue-specific expression of a gene. While each enhancer region for a gene is present in all cells, specific transcription factors that can bind to it to promote transcription are only made in certain specific cells. This allows for regulation of where the gene is expressed (spatial regulation of expression) Figure 3.7 Enhancer region modularity Specific transcription factors may also be made in a given cell only at specific times (for example, specific times during development, or in response to a signal), allowing temporal regulation of gene expression (regulating when genes are expressed). Insulators Insulators: Found between an enhancer and a promoter for a nontarget gene – allow some enhancer–promoter interactions and block others – some may prevent chromatin spreading in the region of the gene – cis-acting transcription regulatory elements Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Variants of DNA-binding proteins that serve as transcription factors include -Helix-turn-helix (HTH) -Zinc-finger -Leucine zipper (ZIP) http://www.mun.ca/biology/desmid/ brian/BIOL2060/BIOL2060-23/ CB23.html Copyright © 2019 Pearson Education, Inc. All Rights Reserved Protein interactions of TF involve many activator and coactivator proteins Copyright © 2019 Pearson Education, Inc. All Rights Reserved Formation of RNA Pol II Initiation Complex Formation of RNA Pol II initiation complex to begin transcription General (basal) transcription factors (GTF) – Proteins – Required at promoter to initiate basal or enhanced levels of transcription – Assembly of proteins in specific order forms pre- initiation complex (PIC) – this is necessary to initiate transcription – PIC provides platform for RNAP II to recognize transcription start sites Copyright © 2019 Pearson Education, Inc. All Rights Reserved Core Promoter TFIID GTFs that assist RNAP II at a core promoter: TFIIA,TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and a large multi-subunit complex called Mediator Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Core Promoter TFIID Core promoter TFIID – general transcription factor needed to initiate transcription of genes in eukaryotes – Assists RNA Pol II association with core promoter – Comprised of TBP and TAF transcription factors – First step in forming PIC— binding of TFIID to TATA box via TATA binding protein (TBP) Copyright © 2019 Pearson Education, Inc. All Rights Reserved Mechanism of Transcription Activation and Repression Transcription activators and repressors bring changes to RNA Pol II transcription – DNA looping delivers specific transcription factors (activators, repressors) and general transcription factors to promoter vicinity – Recruitment model: enhancers and silencer elements act as donors; affect regulatory proteins at gene promoters – Specific transcription factors can also regulate gene expression by recruiting proteins that modify chromatin Copyright © 2019 Pearson Education, Inc. All Rights Reserved Coactivators and Enhanceosome Coactivators – Interact with proteins and enable activators to make contact with promoter-bound factors – Coactivators form complex “enhanceosome” Enhanceosome – Interacts with transcription complex – Activator specific transcription factors at enhancer elements promote PIC assembly and help recruit RNA pol II. – Repressor specific transcription factors at silencer elements decrease rate of PIC assembly and RNA Pol II release Copyright © 2019 Pearson Education, Inc. All Rights Reserved Coactivators and Enhanceosome Copyright © 2019 Pearson Education, Inc. All Rights Reserved 17.7 ENCODE Data Are Transforming Our Concepts of Eukaryotic Gene Regulation Copyright © 2019 Pearson Education, Inc. All Rights Reserved Section 17.7 ENCODE ENCODE: Encyclopedia of DNA Elements Project – Goal: Identify all functional DNA sequences and determine how elements regulate expression Discovery by ENCODE – More than 80 percent of human genome contains regulatory elements – (once considered “junk DNA”) Copyright © 2019 Pearson Education, Inc. All Rights Reserved Section 17.7 Many Disease-Associated Genome Variations Affect Regulatory Regions Genome-wide association studies (GWAS) – Identify genetic variations that are significantly enriched in the genomes of patients with a particular disease and are not found in those without the disease Many GWASs evaluate single base-pair changes or single nucleotide polymorphisms (SNPs) Copyright © 2019 Pearson Education, Inc. All Rights Reserved  Control of Gene Expression in Eukaryotes: Post-transcriptional regulation Control of gene expression often involves the control of transcription initiation. But gene expression can also be controlled after transcription, with mechanisms such as: – RNA interference (silencing) – mRNA degradation (regulation of stability) – alternative splicing of the primary mRNA transcript – Regulation of translation and post-translational 18.1 Regulation of Alternative Splicing Determines Which RNA Spliceforms of a Gene are Translated Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved  Section 18.1 Alternative Splicing Alternative splicing of mRNA primary transcript – Generates different forms of mRNA from identical pre-mRNA Expression of one gene gives rise to numerous proteins with similar or different functions – Increases number of proteins made from one gene called isoforms Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved  Posttranscriptional Regulation Introns are spliced out of pre-mRNA (the primary transcript) to produce the mature mRNA that is translated. Alternative splicing using different processes such as recognizing different splice sites in different tissue types and causes different mature mRNAs to be produced FROM THE SAME GENE The mature mRNAs in each tissue possess different exons, resulting in different polypeptide products from the same gene. These different polypeptides (proteins) that are produced through alternative splicing of pre-RNA from a gene are called isoforms. Campbell Biology 11th ed Section 18.1 Types of Alternative Splicing (7 of 7) Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Alternative splicing – Number of proteins cell can make (proteome) is not directly related to number of genes in genome – At least two-thirds of protein-coding genes in humans undergo alternative splicing and can produce different isoforms of a protein product Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.1 Alternative Splicing (4 of 4) Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.1 Regulation of Alternative Splicing Splicing enhancers and splicing silencers – Cis-acting sequences that regulate alternative splicing SR proteins bind to splicing enhancers and activate splicing by recruiting spliceosome components Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a class of proteins that bind splicing silencers and inhibit splicing Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.1 Sex Determination in Drosophila Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved  Spliceopathies: Mutations that affect regulation of splicing and contribute to several genetic disorders – Examples: – Myotonic dystrophy – Spinomuscular atrophy (SMA) Current Opinion in Cell Biology 2018, 52:96–104 18.2 Gene Expression is Regulated by mRNA Stability and Degradation Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved  Section 18.2 Control of mRNA Stability Steady-state level of mRNA – Amount of mRNA in cell available for translation – Determined by combination of transcription and mRNA degradation rates – Lifetime of mRNA varies; it is eventually degraded Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.2 Pathways of Degradation 3 types of processes promote degradation of mRNA Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.2 Pathways of Degradation 3 types of processes promote degradation of mRNA 1. Exoribonuclease enzymes shorten length of poly-A tail called Deadenylation-dependent decay 2. Decapping enzymes remove 7-methylguanine cap m RNA now unstable 3. Endonuclease cleaves mRNA internally Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved 18.3 Noncoding RNAs Play Diverse Roles in Posttranscriptional Regulation Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.3 RNA Interference RNA interference (RNAi) – Sequence specific posttranscriptional regulation – Short RNA molecules regulate gene expression in cytoplasm of plants, animals, and fungi; repress translation and trigger mRNA degradation Phenomena known as RNA-induced gene silencing Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.3 Molecular Mechanisms (2 of 2) Molecular mechanisms of RNA- induced gene silencing siRNA and micro RNAs – Short, double-stranded ribonucleotides – siRNAs (small interfering): Arise in cell due to virus infection (or intentional experimentation in the lab) — produce double-stranded RNA, which is recognized and cleaved by Dicer – micro RNAs: Noncoding RNAs that negatively regulate gene expression Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.3 RNAi in Biotechnology RNAi (RNA interference) in biotechnology – RNAi studied in lab – Developed as pharmaceutical agent – Therapeutic RNAi attacks diseases caused by overexpression of specific gene or normal expression of abnormal gene product – Potential use of RNAi in diagnosis of cancers – RNAi reduces severity of infections by viruses such as HIV, influenza, and polio Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved 18.4 mRNA Localization and Translation Initiation are Highly Regulated Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.4 Cytoplasmic Polyadenylation (1 of 2) mRNAs are not always translated immediately but stored for translation at a later time or in response to certain cues. Thus regulation of translation initiation is also a way to regulate gene expression in eukaryotes. Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Section 18.4 Cytoplasmic Polyadenylation (1 of 2) mRNAs are not always translated immediately but stored for translation at a later time or in response to certain cues. Thus regulation of translation initiation is also a way to regulate gene expression in eukaryotes. Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved 18.5 Posttranslational Modifications Regulate Protein Activity Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved  Post-translational regulation: Protein modifications Proteins are produced and degraded continually in the cell. Protein activity, location within the cell, or degradation can be regulated through a variety of modifications AFTER the protein is produced…. Section 18.5 Regulation of Proteins by Phosphorylation Phosphorylation is the most common type of posttranslational modification – Kinases catalyze the addition of a phosphate group to serine, tyrosine, or threonine amino acid side chains. – Phosphatases are enzymes that remove phosphates Phosphorylation usually induces conformational changes and therefore functional changes in proteins It may also regulate WHERE in the cell the protein is located (e.g., nucleus vs. cytoplasm) Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Regulation of nuclear import of transcription factors Induction of immediate-early genes by ERK  Section 18.5 Ubiquitin-Mediated Protein Degradation (1 of 2) A protein can be targeted for degradation by covalently modifying it with ubiquitin Ubiquitinated proteins are recognized and broken down by the proteasome, a multi-subunit protein complex with protease (protein cleaving) activity. Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved Copyright This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. 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