Photoaffinity labelling with an ATP analog of the N-terminal peptide of myosin.

Photoaffinity labelling of tryptic and chymotryptic heavy meromyosin with 3′O-3-N-(4-azido-2-nitrophenyl) amino]propionyl-adenosine 5′-triphosphate (arylazido-β-alanine ATP) resulted in incorporation of radioactivity and inhibition of the ATPase activity. ATP prevented the reaction with the photoaffinity label, as shown by the lack of incorporation of ³H and intact ATPase activity. On the tryptic digestion of either type of photoaffinity labeled HMM the label was found in a 25K peptide identifiable with the N-terminus of the myosin heavy chain (Lu et al., Fed. Proc. $\text{37}$ 1695 1978). The results are discussed in the light of previous localization of the reactive thiol groups, SH-1 and SH-2 (Balint et al., Arch. Biochem. Biophys. $\text{190}$, 793 1978).