Practice question

Endosomal system is central node of vesicular traffic that integrates endocytic and biosynthetic routes to sort cargo after internalization or secretion. Classical early endosomes originate when clathrin-coated pits invaginate with assistance of AP2 adaptor, epsin and dynamin GTPase scission, or when caveolae containing caveolin internalize, delivering extracellular ligands, nutrient receptors such as transferrin receptor, signaling receptors like EGFR, and fluid-phase markers into cell. These primary vesicles rapidly lose coat through auxilin and Hsc70 and fuse homotypically via Rab5-GTP, EEA1 coiled-coil tether and phosphatidylinositol 3-phosphate to generate larger early sorting endosomes. However endosomes also receive substantial and essential input from trans-Golgi network, TGN, where newly synthesized lysosomal hydrolases tagged with mannose-6-phosphate and lysosomal membrane proteins LAMPs are sorted into clathrin and AP1, GGA vesicles that traffic and fuse with late endosomes marked by Rab7. This dual supply merges surface-derived endocytic material with Golgi-derived enzymes, ensuring lysosomes acquire complement of acidic hydrolases while plasma membrane cargo proteins are efficiently recycled via Rab4 and Rab11 routes. Thus endosomes represent convergence of plasma membrane-derived and TGN-derived pathways, with Golgi contributions indispensable for maturation, acquisition of degradative capacity and eventual fusion with lysosomes.