Cell Biophysiology Overview

Cell Biophysiology - Learning Objectives

• Describe protein synthesis
• State the fate of proteins synthesized in the rough endoplasmic reticulum (rER) and those synthesized by free ribosomes
• Explain the "signal hypothesis" in protein synthesis
• List factors determining the fate of newly synthesized proteins
• Explain the function of the unfolded protein response (UPR) and its importance to the cell
• Compare protein degradation mechanisms using proteasomes or lysosomes
• Explain the cytoskeleton's role in cell shape, organelle positioning, intracellular trafficking, and movement
• Compare the characteristics and functions of microfilaments, microtubules, and intermediate filaments
• Explain treadmilling of actin filaments and dynamic instability of microtubules
• Explain how cytoskeletal dynamics control cellular movement
• Identify and correlate organelles with their functions

Chromatin structure

• Chromatin is DNA bound to histones and other proteins
• Chromatin exists in two forms:
  • Heterochromatin: Condensed chromatin, transcriptionally inactive, appears intensely stained
  • Euchromatin: Dispersed, unpacked chromatin, often actively transcribed, appears lightly stained
• Nucleolus: Site of ribosomal RNA (rRNA) transcription and ribosome production; appears intensely stained

Constitutive and Facultative Heterochromatin

• Constitutive heterochromatin: Repetitive, stays condensed throughout the cell cycle
• Facultative heterochromatin: Non-repetitive, can lose condensed state and become transcriptionally active

Centrosome and Centriole

• Centrosome: Microtubule organizing center in animal cells, consisting of:
  • Centrioles: Orthogonal pairs of centrioles
  • Pericentriolar material (PCM): Amorphous mass of proteins; responsible for microtubule anchoring. Cylindrical structure, ~200 nm in diameter, composed of nine triplets of microtubules

Polyribosomes (Polysomes)

• Multiple ribosomes bound to a single mRNA molecule
• Simultaneously translate mRNA into many protein copies

Polyribosomes: Free and ER-Bound

• Free polyribosomes synthesize cytosolic and cytoskeletal proteins and proteins for import into the nucleus, mitochondria, and peroxisomes
• ER-bound polyribosomes synthesize proteins for incorporation into membranes, storage in lysosomes, and secretion from the cell

Protein Synthesis - Signal Hypothesis

• Signal hypothesis describes protein translocation into the endoplasmic reticulum (ER)
• Signal peptide: Sequence at the N-terminus of a protein targeted for the ER
• Signal recognition particle (SRP): Binds to the signal peptide
• SRP-receptor binds to the SRP, halting translation
• Ribosome-translocator complex: Ribosome docks on the ER membrane
• Signal peptidase: Removes the signal peptide
• Chaperones: Assist with proper polypeptide folding in the ER cisterna

ER-Associated Degradation (ERAD)

• ERAD pathway degrades misfolded or unfolded proteins
• Ubiquitin proteins are conjugated to misfolded proteins
• Degraded by proteasomes

Unfolded Protein Response (UPR)

• UPR triggers when unfolded proteins accumulate in the ER
• Adjusts the number of proteins that need to be folded
• Increases endoplasmic reticulum's protein folding ability
• Removes proteins that take longer to fold

Ubiquitin-Proteasome Pathway

• Ubiquitin-protein ligation: Ubiquitin attached to a protein via enzymes
• Recognition of the ubiquitin-conjugated protein by the 26S proteasome
• Degradation of the ubiquitin-conjugated protein by the 26S proteasome
• Release and recycling of ubiquitin
• Disassembly of the polyubiquitin chain

Self-Study Slides

• Organelle identification and correlating with function