SPECIFIC LIPID-PROTEIN INTERACTIONS CHARACTERIZE 3 POPULATIONS OF CLATHRIN-COATED VESICLES INVOLVED IN THE LDL RECEPTOR TRAFFIC

We have previously isolated 3 different populations of clathrin coated vesicles (CCV) involved in the LDL-receptor traffic in bovine adrenal cortex. We now show that each CCV type contains the transferrin-R and the CI-MPR, therefore, they provide a good model for studying the mem brane organization that may govern their targeting in one of the biosy nthetic, endocytic and/or recycling pathways. Transferrin - prototype of recycling ligand -, and alpha adaptin, dynamin and the 100 kDa phos phatidylinositol-3-kinase subunit - of the trafficking machinery - wer e mainly detected in only 2 of the vesicle populations which could be involved in the endocytic/recycling pathway. The third population whic h contained larger amounts of gamma adaptin and do no carry transferri n could be involved in the biosynthetic pathway. The vesicle lipid pat tern and the saturation of their fatty acyl chains were analyzed and c onfirmed these results. The nature of the interactions between vesicle components was then determined using several classes of detergents. O nly non ionic ones could solubilize the LDL-R in a complex with either alpha or gamma adaptin. In contrast, they dissociated clathrin or bet a-beta'-adaptins. Taken together these results prompt use to suggest a n integrated model for targeting in membrane traffic. Besides specific targeting machinery, lipids would play a key modulatory role. At each step in the membrane traffic, the proteins which carry multiple targe ting signals would interact transiently with a specific set of lipids. This would result in the exposure of the appropriate targeting signal s which could now become recognized by the proper targeting machinery.