Identification and characterization of novel clathrin adaptor-related proteins

We have identified a human similar to 87-kDa protein, designated as gamma 2-adaptin, that is similar to gamma-adaptin (called gamma 1-adaptin in this paper), a large chain of the AP-1 clathrin-associated adaptor complex, not only in the primary structure (60% amino acid identity) but also in the domain organization. Northern blot analysis has shown that its mRNA is expressed in a variety of tissues. Analysis using a yeast two-hybrid system has revealed that, similarly to gamma 1-adaptin, gamma 2-adaptin is capable of interacting not only with the sigma 1 chain (called as sigma 1A in this paper), the small chain of the AP-1 complex, but also with a novel sigma 1-like protein, designated as sigma 1B, which shows an 87% amino acid identity to sigma 1A; and that, unlike gamma 1-adaptin, it is unable to interact with beta 1-adaptin, another large chain of the AP-1 complex. Immunofluorescence microscopy analysis has revealed that gamma 2-adaptin is localized to paranuclear vesicular structures that are not superimposed on structures containing gamma 1-adaptin. Furthermore, unlike gamma 1-adaptin, gamma 2-adaptin is recruited onto membranes in the presence of a fungal antibiotic, brefeldin A. These data suggest that gamma 2-adaptin constitute a novel adaptor-related complex that participates in a transport step different from that of AP-1.