Procathepsin L Self‐Association as a Mechanism for Selective Secretion

The lysosomal cysteine pro‐protease procathepsin L was enriched in dense vesicles detectable when microsomes prepared from wild‐type or transformed mouse fibroblasts were resolved on sucrose gradients. These dense vesicles did not comigrate with proteins characteristic of the endoplasmic reticulum, Golgi, endosomes or lysosomes. When gradient fraction vesicles were lysed at acidic pH in the presence of excess mannose 6‐phosphate to prevent binding to mannose phosphate receptors, the majority of the procathepsin L was associated with the membrane, not the soluble, fraction. Immunogold labeling of procathepsin L in thin sections of cells or gradient fractions, using antibodies directed against the propeptide to avoid detection of the mature enzyme in dense lysosomes, revealed that the proenzyme was concentrated in dense cores localized in small vesicles near the plasma membrane and in multivesicular bodies. Consistent with the density of the gradient fraction and the electron density of the cores, yeast two‐hybrid assays indicated the proenzyme could bind itself but could not interact with the aspartic proprotease procathepsin D. The data suggest that in mouse fibroblasts procathepsin L may self‐associate into aggregates, initiating the formation of dense vesicles that could mediate the selective secretion of procathepsin L independent of mannose phosphate receptors.