PURIFICATION OF A NEW CLATHRIN ASSEMBLY PROTEIN FROM BOVINE BRAIN COATED VESICLES AND ITS IDENTIFICATION AS MYELIN BASIC-PROTEIN

The multimeric clathrin assembly proteins AP-1 and AP-2 with molecular masses of similar to 270 kDa and the monomeric clathrin assembly prot eins AP(180) and auxilin with molecular masses of similar to 90 kDa ca talyze the assembly of clathrin into artificial clathrin baskets under physiological conditions. We have now identified a much smaller simil ar to 20-kDa clathrin assembly protein in 0.5 M Tris, pH 7.0, extracts of bovine brain coated vesicles and purified it to near homogeneity. A polyclonal antibody against this protein did not cross-react with an y of the other assembly proteins, and sequencing data suggest that thi s new protein is similar or identical to myelin basic protein (MBP). A t a molar ratio of 3 molecules per clathrin triskelion, MBP catalyzes polymerization of clathrin into artificial baskets that appear structu rally similar to the baskets assembled by the other assembly proteins. In addition, like the other baskets, the clathrin-MBP baskets are unc oated by hsp70. MBP represents a significant fraction of the total ass embly protein activity present ent in 0.5 M Tris, pH 7.0, extracts of coated vesicles. It is not clear if it acts as an assembly protein in vivo, but because it is well characterized and easily available, MBP w ill be a useful protein to investigate the mechanism of clathrin assem bly and disassembly in vitro.