Abstract
A number of lysosomal glycoproteins exist which are membrane associated and do not contain the mannose-6-phosphate lysosomal targeting signal which sorts soluble lysosomal hydrolases to the lysosome in the Golgi apparatus. These include lysosomal acid phosphatase (LAP), which is targeted as a transmembrane protein to the lysosome where it is cleaved to release the soluble enzyme, and a number of lysosomal membrane associated glycoproteins including the homologous families of LAMP-1 and LAMP-2 as well as other smaller molecular weight glycoproteins including LIMP II and CD63. The cytoplasmic domain of LAP, LAMP-1 and LIMP II is capable of targeting the ectodomain of non-lysosomal glycopoteins to the lysosome. Except for LIMP II, the lysosomal membrane glycoproteins contain a Gly-Tyr-X-X-hydrophobic amino acid lysosomal targeting motif in their cytoplasmic domains. Studies of LAP, LAMP-1 and LAMP-2 have generated conflicting data as to the biosynthetic targeting pathway of these proteins and, in particular, as to whether it includes the plasma membrane.
