THE BETA-1-SUBUNIT AND BETA-2-SUBUNIT OF THE AP COMPLEXES ARE THE CLATHRIN COAT ASSEMBLY COMPONENTS

The beta1 and beta2 subunits are the closely-related large chains of t he trans-Golgi network AP-1 and the plasma membrane AP-2 clathrin-asso ciated protein complexes, respectively. Recombinant beta1 and beta2 su bunits have been generated in Escherichia coli. It was found that, in the absence of alt the other AP subunits, beta1 and beta2 interact wit h clathrin and drive the efficient assembly of clathrin coats. In addi tion, beta2 subunits and AP complexes compete for the same clathrin bi nding site. The appearance of the clathrin/beta coats is the same as t he barrel-shaped structures formed with native AP complexes. It is pro posed that the principal function of the beta subunits is to initiate coat formation, while the remaining subunits of the AP complexes have other roles in coated pit and coated vesicle function.