PROTEOLYSIS AND FUSION OF LOW-DENSITY-LIPOPROTEIN PARTICLES INDEPENDENTLY STRENGTHEN THEIR BINDING TO EXOCYTOSED MAST-CELL GRANULES

Contact between low density lipoproteins (LDL) and exocytosed mast cel l granules, the ''granule remnants,'' leads to binding of LDL to the g ranule remnants via ionic interactions between the apolipoprotein B-10 0 (apoB-100) component of LDL and the heparin proteoglycan component o f the granule remnants. Upon incubation at 37-degrees-C, the heparin p roteoglycan-bound apoB-100 is progressively proteolyzed by remnant chy mase and carboxypeptidase A, which are also bound to the heparin prote oglycans. Thereupon, the LDL particles fuse, and their binding to the granule remnants strengthens, as defined by the decreased ability of N aCl to release LDL from the remnants. We now have examined separately the effects of proteolysis and fusion on LDL binding. Proteolysis with out fusion was induced by lowering the incubation temperature to 15-de grees-C, and proteolysis-independent fusion was induced by treating gr anule remnant-bound LDL with sphingomyelinase in the presence of prote ase inhibitors. It was found that degradation of the heparin proteogly can-bound apoB-100, even without accompanying particle fusion, increas ed the strength of LDL binding to the granule remnants, suggesting exp osure of buried heparin binding regions of apoB-100. When such proteol yzed LDL particles were allowed to fuse, the strength of their binding to the granule remnants increased still further, probably because of an increase in the number of apoB-100 fragments in the enlarged partic les. Proteolysis-independent fusion, induced by sphingomyelinase treat ment of granule remnant-bound LDL, also increased the strength of bind ing. The results show that proteolytic degradation and fusion, the two modifications of granule remnant-bound LDL subsequent to action by ch ymase and carboxypeptidase A of the granule remnants, represent two se parate mechanisms by which LDL particles become tightly bound to the h eparin proteoglycans of exocytosed mast cell granules. Since the forma tion of an atheroma, the hallmark of atherosclerosis, is characterized by accumulation in the proteoglycan matrix of the arterial intima of extracellular lipid droplets resembling the fused LDL particles on the granule remnant surfaces, the modifications of LDL described in this study may provide a clue to the actual processes by which the lipid dr oplets are anchored to the arterial intima.