The NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta(3) integrin A isoform

beta (3) integrin adhesion molecules play important roles in wound repair a nd the regulation of vascular development and three beta (3) integrin isofo rms (beta (3)-A, -B, -C) have been described so far. Surface expression of beta (3) integrins is dynamically regulated through internalization of beta (3) integrins, however, the molecular mechanisms are understood incomplete ly, To evaluate the role of the cytoplasmic domain of beta (3) integrins fo r internalization, we have generated single chain chimeras with variant and mutated forms of beta (3) cytoplasmic domains, Upon transient transfection into chinese hamster ovary cells, it was found that the beta (3)-A chimera had strongly reduced cell surface expression compared with the correspondi ng beta (3)-B, or beta (3)-C fusion proteins, or the tail-less constructs, whereas steady state levels of all chimeras were near identical, Studies em ploying cytoplasmic domain mutants showed that the NITY motif at beta (3)-A 756-759 is critical for plasma membrane expression of beta (3)-A Furthermo re, delivery of beta (3)-A to the cell surface was specifically modulated b y the cytoplasmic protein beta (3)-endonexin, a previously described intrac ellular protein, Coexpression of the native, long form of beta (3)-endonexi n, which does not interact with the beta (3) tail, acted as a dominant nega tive inhibitor of beta (3)-A-internalization and enhanced steady-state surf ace expression of the beta (3)-A-chimera, Furthermore, anti-beta (3) antibo dy-induced internalization of the native beta (3) integrin (alpha (IIb)beta (3)) was dramatically reduced for the Tyr(759)-Ala substitution mutant alp ha (IIb)beta (3) (Y759A) and expression of the long isoform of beta (3)-end onexin substantially decreased the internalization of wild-type alpha (IIb) beta (3) Thus, the NITY motif of the beta-chain cytoplasmic domain is invol ved in stimulated internalization of the beta (3) integrin A isoform and be ta (3)-endonexin appears to couple the beta (3)-A isoform to a specific rec eptor-recycling pathway.