Adenosine deaminase is a specific partner for the Grb2 isoform Grb3-3.

Grb3-3 is an isoform of Grb2, thought to arise by alternative splicing, that lacks a functional SH2 domain but retains functional SH3 domains, which allow interaction with other proteins through binding to prolinerich sequences. Several evidences suggest that besides common partners for Grb2 and Grb3-3, specific targets could exist. In order to find specific partners for Grb3-3, we have screened a human cDNA library by the yeast two-hybrid system with Grb3-3 as a bait. We have identified adenosine deaminase, an enzyme involved in purine metabolism whose deficiency is associated with severe combined immunodeficiency, as a Grb3-3 binding protein that is not able to bind to Grb2. This interaction has been confirmed in vitro with GST fusion proteins and in vivo by coimmunoprecipitation experiments in NIH3T3 cells stably transfected with Grb3-3. The functional significance of this finding is discussed.