Figure 4
FLAG-MHCK-C is activated by autophosphorylation. The peptide substrate MH-1 is a 16 residue peptide that corresponds to the mapped myosin II phosphorylation site at position 2029 of MHC. A. Phosphorylation of MH-1 by FLAG-MHCK-C occurs with a reproducible lag phase (open symbols), similar to the lag seen with myosin II as the substrate. Preincubation of FLAG-MHCK-C with MgATP eliminates the lag phase (closed symbols). Phosphorylation is plotted in terms of moles Pi transferred as fraction of total moles MH-1 peptide in reaction. B. Expanded plot of early time points of the same experiment. Bars represent S.E.M., n = 3. C. Addition of myosin II to FLAG-MHCK-C autophosphorylation reactions does not accelerate MHCK-C autophosphorylation.