Localization of two lysosomal proteinases on the external surface of the lysosomal membrane

Abstract

Two enzymes present in rabbit liver extracts that catalyze the limited proteolysis of rabbit liver fructose 1,6-bisphosphatase (EC 3.1.3.11) and rabbit liver fructose 1,6-bisphosphate aldolase (EC 4.1.2.13) have now been identified as lysosomal proteinases. This identification was based on their cosedimentation with cathepsin C (EC 3.4.14.1) in the particulate fractions isolated from livers of normal and Triton WR1339-injected rabbits. These proteinases, designated Fru-P 2ase-converting enzyme 2 (CE2) and cathepsin M, respectively, are partially associated with lysosomal membranes. The membrane-bound activity is selectively destroyed by incubation of the intact lysosomes with soluble or immobilized trypsin. The results suggest that the CE2 and cathepsin M activities are located in part on the outer lysosomal surface and that modification of cytosolic proteins by these proteinases may occur without entry of the proteins into the lysosomes.