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 membrane organization that may govern their targeting in one of the biosynthetic, endocytic and/or recycling pathways. Transferrin--prototype of recylcing ligand--, and alpha adaptin, dynamin and the 110 kDa phosphatidylinositol-3-kinase subunit--of the trafficking machinery--were mainly detected in only 2 of the vesicle populations which could be involved in the endocytic/recycling pathway. The third population which contained larger amounts of gamma adaptin and do not carry transferrin could be involved in the biosynthetic pathway. The vesicle lipid pattern and the saturation of their fatty acyl chains were analyzed and confirmed these results. The nature of the interactions between vesicle components was then determined using several classes of detergents. Only non ionic ones could solubilize the LDL-R in a complex with either alpha or gamma adaptin. In contrast, they dissociated clathrin or beta-beta' adaptins. Taken together these results prompt us to suggest an integrated model for targeting in membrane traffic. Besides specific targeting signals carried by cargo proteins and recognized by proteins from the coat and the cytosolic trafficking machinery, lipids would play a key modulatory role. At each step in the membrane traffic, the proteins which carry multiple targeting signals would interact transiently with a specific set of lipids. This would result in the exposure of the appropriate targeting signals which could now become recognized by the proper targeting machinery.