Protein Synthesis Part 5 PDF

Protein Synthesis part 5 2B03 Images retrieved from “Biochemistry, the molecular basis of life”, McKee, 7th edition or my own Posttranslational modifications Prepare each molecule for its functional role and/or foldin...

Protein Synthesis part 5 2B03 Images retrieved from “Biochemistry, the molecular basis of life”, McKee, 7th edition or my own Posttranslational modifications Prepare each molecule for its functional role and/or folding into its proper conformation Direct the protein to a specific location (targeting) There are numerous posttranslational modifications in eukaryotes Glycosylation Addition of a sugar molecule to the protein Wide variety of eukaryotic proteins are glycosylated for structural purposes Example: proteoglycans → glycosylated proteins serve as a cushion and hydration source for tissue and organs Methylation Addition of methyl groups to proteins Several purposes in eukaryotes, including marking proteins for repair or degradation or changing their cellular function Another example is methylation of nucleosomal histones at specific lysine or arginine residues→ can affect chromatin conformations and facilitate transcription  Proteolytic cleavage Common regulatory mechanism in eukaryotic cells Examples are the removal of Methionine and signal peptides This mechanism is used to convert inactive precursor proteins (proproteins) to their active forms Example: insulin Disulfide bond formation Disulfide bonds are typically present in secretory proteins (insulin) and certain membrane proteins Protein disulfide isomerase (PDI) is the enzyme that catalyzes this process PDI can also act as a chaperone by rescuing misfolded proteins Diabetes and protein misfolding Diabetes is a metabolic disorder characterized by chronically elevated blood glucose levels (hyperglycemia) Insulin is a peptide hormone that is essential for the control of blood glucose levels (reduces glucose levels in the blood) Insulin is synthesized in the beta cells in the pancreas When we have chronic hyperglycemia, the demand of insulin is increased  Diabetes and protein misfolding An increase in insulin demand can cause an excess of misfolded proteins, a condition known as Endoplasmic Reticulum Stress Y Properly folded Misfolded Endoplasmic Reticulum Stress Adaptive Unfolded Protein Response (UPR) Apoptotic UPR Increased folding capacity Increased pro-apoptotic molecules ↑GRP78, PDI Increased ER Associated Degradation (ERAD) CELL SURVIVAL CELL DEATH An example from my graduate research project Immunofluorescence to detect GRP78 in the pancreas Protein targeting In eukaryotes we have two mechanisms: Transcript localization Signal peptides Transcript localization Various sets of proteins bind to the mature mRNA and can direct it to the Endoplasmic Reticulum (i.e. exon junction complex) These sets of proteins bind to the mature mRNA to the 3’-UTR sequence (precedes the stop codon) and can determine: The specific destination in the rER The degree to which mRNA will be translated: although we don’t know the exact mechanisms, some cells can selectively degrade mRNA in specific areas of the cell and not in others→ specific localization of a protein in the cell Image retrieved from: https://www.ncbi.nlm.nih.gov/books/NBK563126/figure/article-28660.image.f1/ Targeting through signal peptides Explains how polypeptides destined for secretion, for use in the plasma membrane, or membranous organelles are targeted to their location by sorting signals→ signal peptides (SPs) SP: short peptide sequences located at the N-terminal SPs act as “zip codes” enabling the protein reach the specific target location  Targeting through signal peptides Image retrieved from: https://studiousguy.com/smooth-endoplasmic-reticulum- structure-functions-and-diagram/ Translocation of secretory (or membrane) proteins N-terminal signal peptide is recognized by a signal recognition particle (SRP) The ribosome-protein complex is transferred to an SRP receptor on the ER SRP is released and the nascent protein is inserted into the translocon, a membrane-bound protein conducting channel Translocation of secretory (or membrane) proteins The signal sequence is cleaved Protein synthesis continues, protein folding is facilitated by Hsp70 chaperones The folded protein is released, and ribosome departs from translocon Please note: BiP is aka GRP78, the molecular chaperone that belongs to the family of Hsp70 ☺ Translation control mechanisms Many levels of control Aberrant mRNA: mRNA is processed and checked by specific protein complexes (i.e. exon junction complex in the first round of translation) mRNA degradation machineries are activated if the mRNA is aberrant mRNA stability: transcription of a gene does not guarantee that it will be translated Stability is influenced by how abundant the mRNA is, the presence of sequences that confer resistance to nuclease action, the presence of RNA-binding proteins to certain sequences mRNA decay mRNA decay describes the process by which mRNA is degraded mRNA decay happens when enough protein is produced (or mRNA is aberrant) and occurs in the mRNA processing bodies (P bodies) mRNA decay involves deadenylation, decapping, and nucleotide degradation mRNA decay Specific enzymes called deadenylases shorten the poly (A) tail deadenylases The loss of the poly(A) tail weakens mRNA circularization  mRNA decay Degradation of the polyA tails trigger the removal and digestion of the 5’ cap (decapping) Deadenylases The decapped mRNA is then degraded from the 5′ end by 5′-3′ exoribonuclease 1 (XRN1) or from the 3′ end by the exosome complex.