Cellular membranes enter the lysosomal compartment by endocytosis, phagocytosis, or autophagy. Within the lysosomal compartment, membrane components of complex structure are degraded into their building blocks. These are able to leave the lysosome and can then be utilized for the resynthesis of complex molecules or can be further degraded. Constitutive degradation of membranes occurs on the surface of intra-endosomal and intra-lysosomal membrane structures. Many
integral membrane proteins are sorted to the inner membranes of endosomes and lysosome after ubiquitinylation. In the lysosome,
proteins are degraded by
proteolytic enzymes, the
cathepsins.
Phospholipids originating from
lipoproteins or cellular membranes are degraded by
phospholipases. Water-soluble
glycosidases sequentially cleave off the terminal
carbohydrate residues of
glycoproteins,
glycosaminoglycans, and
glycosphingolipids. For
glycosphingolipids with short
oligosaccharide chains, the additional presence of membrane-active lysosomal
lipid-
binding proteins is required. The presence of
lipid-
binding proteins overcomes the phase problem of water soluble
enzymes and
lipid substrates by transferring the substrate to the degrading
enzyme or by solubilizing the internal membranes. The
lipid composition of intra-lysosomal vesicles differs from that of the plasma membrane. To allow at least
glycosphingolipid degradation by
hydrolases and activator
proteins, the
cholesterol content of these intraorganellar membranes decreases during endocytosis and the concentration of
bis(monoacylglycero)phosphate, a stimulator of
sphingolipid degradation, increases. A considerable part of our current knowledge about mechanism and biochemistry of lysosomal
lipid degradation is derived from a class of human diseases, the
sphingolipidoses, which are caused by inherited defects within
sphingolipid and
glycosphingolipid catabolism.