Fig. 1. ERK5 is activated upon differentiation in C₂C₁₂ cells. C₂C₁₂ myoblasts were plated at a density of 2.5 × 10⁵ on 10 cm dishes in DMEM/10%FBS. Twenty-four hours later cells were shifted to DMEM/2%HS to induce differentiation. Cell lysates were prepared at different time points as indicated. ERK5 was immunoprecipitated from the lysates using a polyclonal goat anti-ERK5 antiserum. ERK5 activity was measured by an immune complex kinase assay using MBP as a substrate. Kinase assays were performed essentially as described (Ludwig et al., 1996). Equal loading of kinases was assessed by immunoblotting with an ERK5-specific antiserum.