Brain Clathrin: Studies of Its Ultrastructural Assemblies

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Abstract
Bovine brain clathrin purified to a high degree of homogeneity showed the presence of accompanying polypeptides of lower MW and assembled into baskets or cages when the pH of its solution was adjusted from pH 7.5 to 6.5. Brief chymotrypsin treatment of this clathrin preparation at pH 6.5 cleaved clathrin-associated proteins rendering clathrin unable to reform baskets. Hydrolysis of clathrin-associated proteins did not affect clathrin''s capacity to polymerize into open lattices or to bind actin and .alpha.-actinin from solution. When open lattices formed, the turbidity of the solution rose to levels that were higher than those produced when cages or baskets were formed by native clathrin. In both native or enzyme-treated clathrin, turbidity increase was inhibited by salts. The addition of certain cytoskeletal-disrupting agents, such as chloropromazine and imipramine, increased clathrin''s turbidity but did not result in formation of baskets. Colchicine and cytohalasin B did not affect turbidity or the assembly of cages. The addition of increasing amounts of vinblastine resulted in the precipitation of larger amounts of clathrin which, after solubilization, retained its ability to polymerize into baskets. Clathrin apparently exists as a complex with other protein molecules, clathrin-associated proteins, that modulate the extent of polymerization and assembly of clathrin into characteristic structures.