Src can regulate carboxy terminal interactions with AFAP-110, which influence self-association, cell localization and actin filament integrity

Abstract

The SH2 and SH3 binding partner AFAP-110 is a tyrosine phosphorylated substrate of Src. AFAP-110 has been hypothesized to link Src to actin filaments, which may contribute to the effects of Src upon actin filament integrity. However, it has been unclear what effect activated Src (Src^527F) has upon AFAP-110 structure or function and whether AFAP-110 plays a role in actin filament integrity. We report here that the carboxy terminal 127 amino acids of AFAP-110 are comprised of an α-helical region that contains a leucine zipper motif. This indicated the potential of AFAP-110 to self-associate. Expression of the carboxy terminus as a fusion protein (GST-cterm) will permit affinity absorption of cellular AFAP-110. The integrity of the α-helical leucine zipper motif in GST-cterm is required for affinity absorption, but binding is not due to a classical leucine zipper interaction. Co-expression of Src^527F, unlike cSrc, will abrogate affinity absorption of AFAP-110 with GST-cterm. These data indicate that Src^527F has affected a change in the carboxy terminal structure that renders AFAP-110 unavailable for affinity absorption. Superose chromatography demonstrate that AFAP-110 will fractionate as a monomer or multimer, indicating AFAP-110 can be detected in a self-associated form in cell lysates. Co-expression of Src^527F resulted in AFAP-110 fractionating with a molecular weight that predicts only a multimeric population. Deletional mutagenesis also indicate a biological role for the carboxy terminus in cellular localization and actin filament integrity. Deletion of the entire carboxy terminal α-helix (84 amino acids) will not permit AFAP-110 to efficiently colocalize with actin filaments or the cell membrane. Deletion of only the leucine zipper region of the carboxy terminal α-helix (44 amino acids) from AFAP-110 (AFAPA^Δzip) demonstrate that both AFAP^Δlzip and actin filaments are repositioned into rosette-like structures, similar to the effects of Src^527F, while co-expression of AFAP-110 with cSrc will not affect actin filaments. These data indicate that AFAP-110 can play an important role in modulating actin filament integrity through carboxy terminal interactions that can be affected by Src^527F.