Dephosphorylation of SP/TP sites in the KIS domain does not affect the association of STEP with ERK2 or its phosphatase activity. Hela cells were transfected with (A) myc-tagged ERK2 and V5-tagged STEP46 S49A/C300S or one of its mutants (S49A/T59E/S72E/C300S, S49E/T59E/S72E/C300S, S49E/T59A/S72A/C300S, S49A/T59A/C300S, S49A/S72A/C300S) and (B) V5-tagged STEP46 S49A or one of its mutants (S49A/T59E/S72E, S49E/T59E/S72E, S49E/T59A/S72A). (A, B) STEP46 was immunoprecipitated using anti-V5 antibody. (A) Immune-complexes were processed for SDS-PAGE and immunoblot analysis with anti-ERK2 antibody to evaluate the association of ERK2 with STEP mutants (upper panel). Blots were reprobed with anti-V5 antibody (lower panel). (B) PTP activity was assayed using pNPP as a substrate. Quantitative measurement of the formation of para-nitrophenolate is represented as mean ± SEM (n = 4).