Concerning a possible mechanism for selective capture of cytoplasmic proteins by lysosomes.

Lysosomes seem to be major agents of degradation of intracellular proteins. There is normally little release of intact proteins from lysosomes to cytoplasm, nor accumulation within lysosomes. As the halflives of cytoplasmic proteins are heterogeneous, their rates of degradation by lysosomes are probably determined by their rates of entry. Therefore, a mechanism for selective uptake of cytoplasmic proteins seems likely. It is suggested that proteins which adsorb to the membranes forming autophagic vacuoles may enter selectively by analogy with the process of adsorptive pinocytosis. Evidence for selective adsorption of rapidly-turning over cytoplasmic proteins to the external membranes of lysosomes, and to lipsomes, consistent with this hypothesis, is presented.