Practice question

Endosomal maturation transforms early sorting compartments into degradative organelles preparing cargo for destruction and nutrient recovery. Late endosomes, also termed multivesicular bodies, marked by switch from Rab5 to Rab7 GTPase, more acidic pH near 5.0 to 5.5 due to increased V-ATPase density, and accumulation of intraluminal vesicles formed by ESCRT machinery, move toward microtubule organizing center along microtubules via RILP and dynein. They progressively acquire lysosomal membrane proteins LAMP1, LAMP2 and soluble hydrolases via transport from trans-Golgi network through mannose-6-phosphate receptors, eventually fusing with terminal lysosomes. Fusion is mediated by tethering complexes HOPS and CORVET bridging membranes, Rab7 effectors RILP and FYCO1 coordinating motility, and SNARE proteins such as Syntaxin 7, Syntaxin 8, VTI1b on late endosome and VAMP8 on lysosome driving lipid bilayer merging through zippering four-helix bundle. This creates a transient endolysosome or mature secondary lysosome where internalized growth factors, LDL-derived cholesterol, and pathogens are degraded by acidic hydrolases cathepsins and lipases, generating amino acids, sugars and lipids effluxed via transporters for reuse, coupling endocytosis to lysosomal catabolism and antigen presentation while avoiding leakage of toxic contents.