in vivo hrdgb2 ha or hrdgb3 ha were coexpressed either with pyk2 or with fak in cos cells following cell lysis hrdgb proteins were immunoprecipitated by anti ha antibodies and the presence of pyk2 or fak in the immunocomplexes was determined by immunoblotting with antibodies against pyk2 or fak respectively the results indicate that both hrdgb2 and hrsdgb3 interact with pyk2 in vivo no interaction however was detected with the related kinase fak suggesting that hrdgbs proteins interact strongly and specifically with pyk2 to explore whether the u2018 pyk2 binding domain u2019 of hrdgbs is sufficient to confer association of those two proteins in vivo a myc tagged version of the hrdgb2 u2018 pyk2 binding domain u2019 was coexpressed either with pyk2 or with fak in cos cells and their interaction was analyzed the results showed that this domain can interact with pyk2 in vivo and therefore represent a separate domain in this family of proteins to confirm the interaction of hrdgb1 and pyk2 in vivo an hemagglutinin tagged rdgb1 and pyk2 were coexpressed in 293 cells the results indicate that hrdgb1 strongly associates with pyk2 association of hrdgb1 with the related kinase fak could not be detected under the same experimental conditions suggesting a strong and specific interaction of hrdgb1 and.