ENDOSOMAL PROTEOLYSIS OF INTERNALIZED PROTEINS

Endosomal proteases have been implicated in the degradation of interna lized regulatory peptides involved in the control of metabolic pathway s and in the processing of intracellular antigens for cytolytic immune responses. Processing in the endocytic vesicles is regulated by chang es in endosomal acidity due to the presence of an ATP-dependent proton pump which modulates protease activity, protein unfolding and recepto r-ligand interactions, A limited number of proteases appear to reside in endosomes which do not contain the full complement of active protea ses capable of completely degrading all internalized polypeptides, Ret ention of some acid hydrolases in endosomes is apparently related to t heir association with undefined endosomal membrane receptors, The limi ted number of proteases and the pH gradient from neutral to acidic (pH 7 to 5) within endosomes make possible a selective and controlled pro cessing environment in comparison to lysosomes, The full set of endo- and exopeptidases that break down proteins to amino acids are active l ater in the pathway in lysosomes.