Chapter 8: Microtubules, Microfilaments & IFs

The system encompasses the Endoplasmic Reticulum (ER), divided into the RER (rough ER) and SER (smooth ER); the RER functions as the primary site for synthesizing secretory, lysosomal, and integral membrane proteins, as well as most cellular lipids, utilizing the signal hypothesis to initiate co-translational translocation via the SRP and the translocon channel. The SER handles non-protein functions like steroid synthesis, detoxification mediated by enzymes such as Cytochrome P450, and the sequestration of calcium ions. Polypeptides synthesized in the RER undergo processing, including enzymatic cleavage, chaperone-assisted folding (using proteins like BiP), and the attachment of N-linked core oligosaccharides assembled on a Dolichol Phosphate carrier. A continuous quality control system ensures proper folding, triggering the destruction of defective proteins through ER-Associated Degradation (ERAD) or activating the Unfolded Protein Response (UPR) if misfolded proteins accumulate. Materials transition from the ER to the Golgi Complex, a central hub consisting of cis, medial, and trans cisternae and the TGN (trans Golgi network), where complex processing occurs, particularly the completion of N-linked and O-linked glycosylation. Transport through the Golgi is best explained by the Cisternal Maturation Model, where cisternae physically progress, while specialized COPI-coated vesicles move resident Golgi enzymes in a retrograde (backward) direction. The TGN acts as the major sorting station, directing secretory materials toward the plasma membrane and lysosomal enzymes, which are tagged with mannose 6-phosphate, into clathrin-coated vesicles mediated by GGA adaptors. Lysosomes are the cell’s acidic digestive organelles, housing numerous acid hydrolases and performing key tasks like breaking down ingested material and recycling old organelles through autophagy, a process similar to the function of large Plant Cell Vacuoles. Directed vesicular transport throughout the endomembrane pathway is mediated by coat proteins (COPII for anterograde ER-to-Golgi transport) and requires the coordination of Rab GTPases, tethering proteins like golgins, and SNARE proteins (v-SNAREs and t-SNAREs) that assemble into four-stranded helical bundles to bring membranes close for fusion, a process often triggered by an increase in calcium ion concentration (Ca2+), resulting in exocytosis. Conversely, the endocytic pathway brings materials into the cell via bulk-phase or selective Receptor-Mediated Endocytosis (RME), exemplified by the uptake of LDL cholesterol, utilizing clathrin-coated vesicles regulated by AP2 adaptors and dynamin. Internalized materials are routed through early and late endosomes (multivesicular bodies) for sorting, recycling, or eventual destruction in lysosomes. Finally, the chapter notes that organelles such as mitochondria, chloroplasts, and peroxisomes import their proteins post-translationally from the cytosol via distinct translocation complexes (e.g., TOM and TIM in mitochondria).