Structural basis for the recognition of tyrosine-based sorting signals by the ?3A subunit of the AP-3 adaptor complex.

ABSTRACT: Tyrosine-based signals fitting the YXXØ motif mediate sorting of transmembrane proteins to endosomes, lysosomes, the basolateral plasma membrane of polarized epithelial cells, and the somatodendritic domain of neurons through interactions with the homologous ?1, ?2, ?3, and ?4 subunits of the corresponding AP-1, AP-2, AP-3, and AP-4 complexes. Previous x-ray crystallographic analyses identified distinct binding sites for YXXØ signals on ?2 and ?4, which were located on opposite faces of the proteins. To elucidate the mode of recognition of YXXØ signals by other members of the ? family, we solved the crystal structure at 1.85 Å resolution of the C-terminal domain of the ?3 subunit of AP-3 (isoform A) in complex with a peptide encoding a YXXØ signal (SDYQRL) from the trans-Golgi network protein TGN38. The ?3A C-terminal domain consists of an immunoglobulin-like ?-sandwich organized into two subdomains, A and B. The YXXØ signal binds in an extended conformation to a site on ?3A subdomain A, at a location similar to the YXXØ-binding site on ?2 but not ?4. The binding sites on ?3A and ?2 exhibit similarities and differences that account for the ability of both proteins to bind distinct sets of YXXØ signals. Biochemical analyses confirm the identification of the ?3A site and show that this protein binds YXXØ signals with 14-19 ?m affinity. The surface electrostatic potential of ?3A is less basic than that of ?2, in part explaining the association of AP-3 with intracellular membranes having less acidic phosphoinositides.

SUBMITTER: Mardones GA 

PROVIDER: S-EPMC3611023 | biostudies-literature | 2013 Mar

REPOSITORIES: biostudies-literature