The isoform-specific stretch of hSos1 defines a new Grb2-binding domain

hSos1 isoform II, defined by the presence of a 15 amino acid stretch in its carboxy-terminal region, exhibits higher Grb2 affinity than hSos1 isoform I. In this study, we investigated the cause for this difference and observe d that, in addition to the four currently accepted Grb2-binding motifs, a n umber of additional, putative SH3-minimal binding sites (SH3-MBS) could be identified. The isoform II-specific 15 amino acid stretch contained one of them, Indeed, me demonstrated by site-directed mutagenesis that these SH3-M BS were responsible for the Grb2 interaction, and we found that C-terminal fragments of the two hSos1 isoforms (lacking the four cannonical Grb2-bindi ng motifs, but containing the SH3-minimal binding sites) were able to bind Grb2, with the isoform II fragment showing higher Grb2 affinity than the co rresponding isoform I fragment. Furthermore, we provide evidence that C-ter minal truncated mutants of either hSos1 isoform, containing only the SH3-mi nimal binding sites, were able to originate in vivo stable complexes with G rb2, Although, Grb2-binding remains higher in both full-length isoforms, co mpared to the C-terminal truncated mutants, these mutants were also able to activate Ras, supporting a potential role of this C-terminal region as neg ative modulator of Sos1 activity. These findings document the existence of a new, functional, SH3-minimal binding site located in the specific stretch of hSos1 isoform II which may be responsible for the increased Grb2 affini ty of this isoform in comparison to isoform I, and for the physiological pr operties differences between both isoforms, Moreover, these SH3-minimal bin ding sites may be sufficient to attain stable and functional hSos1-Grb2 com plexes.