SH2 and SH3 domains have been characterized as functional domains that mediate protein-protein interactions in signal transduction. Recently, the cDNA sequence of a novel Src- and Fyn-binding protein called AFAP-110, for Actin-Filament Associated Protein-110 kDa, was reported. This protein was distinctive in that it is both an SH2 and SH3 binding partner for the non-receptor tyrosine kinases Src and Fyn. Here, we report the characterization of an alternatively processed form of AFAP-110 that encodes an additional 258 base pair (bp) of open reading frame. Transient expression of this full-length clone reveals a molecular mass of 120 kDa. Western blot analysis indicate that a larger 120-kDa variant of AFAP-110 can be detected in brain and is not detectable in any other tissues examined. Northern blot analysis indicate that the novel 258-bp insert can be detected in brain RNA but not chick embryo fibroblast RNA. We propose the name AFAP-120, for Actin Filament-Associated Protein-120 kDa. Expression of the 258-bp novel insert (NINS) as a glutathione S-transferase-encoded fusion protein permits adsorption of a 67-kDa protein from tissue lysates. Deletion analysis of the NINS indicates that the interaction with p67 can be attributed to a proline-rich motif that resembles an SH3-binding motif. We hypothesize that AFAP-120 facilitates interactions in brain between SH2/SH3 signaling proteins and actin filaments and that a proline-rich motif in the NINS may exist to facilitate additional interactions between cellular proteins in brain and actin filaments.

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