Proteins are degraded into a.a. in the lysosome, which then recycles the a.a. for future use. The proteins
that are destined to the lysosome have special proteins that are recognized by cis Golgi. These enzymes
in the cis Golgi recognizes mannose residues, which then serves the purpose of targeting, via the protein
that are found in the trans Golgi, the mannose 6 receptor. This creates a clathrin coated vesicle by ARF,
this recruits AP-1, which forms the base upon which the clathrin coat is added onto. Helped by dynamin.
When the coat falls off, the vesicle fuses with the late endosome, which has a low pH (slightly higher
than lysosome).These pH are maintained by vesicle pumps. M6P receptor loses affinity to M6P, which
then makes the proteins dissociate from the receptor, which allows the receptor to be recycled. Not
100% foolproof, instead of fusing with late endosome could end up in the plasma membrane. This could
cause Ap-2 clathrin vesicle that eventually fuse with late endosome, still end up in the lysosome,
receptor mediated endocytosis.
Many secreted proteins are synthesized into large precursor proteins (pre-proproteins), which are then
cleaved into their final activated form. E.g. Proinsulin - > insulin, converted through multiple cleavage
events involving three proteases, 2 endo and 1 exo at c-terminus. This proteolytic processing takes place
after the vesicles pinches off from the TGN. The proteolytic enzymes takes place in the late endosome or
Receptor mediated endocytosis:
Endocytosis: formation of vesicles at plasma membrane for the internalization of extracellular material.