| Western blot analysis of extracts of rat muscle demonstrated co immunoprecipitation of both IRS 1 and IRS 2 with the skeletal muscle Ca2+ ATPase ( SERCA 1 ) and the cardiac muscle isoform ( SERCA 2 ) . ^^^ Affinity chromatography of phosphopeptide libraries using the glutathione S transferase fusion protein of the C terminus of SERCA 1 indicated a consensus sequence for binding of XpYGSS ; this is identical to potential tyrosine phosphorylation sites at position 431 of human IRS 1 and at position 500 of human IRS 2 . ^^^ |