Serine-324 of myosin's heavy chain is photoaffinity-labeled by 3'(2')-O-(4-benzoylbenzoyl)adenosine triphosphate

Abstract
A portion of the active site of rabbit skeletal myosin near the ribose ring of ATP can be labeled by the photoaffinity analogue 3''(2'')-O-(4-benzoylbenzoyl)adenosine triphosphate (Bz2ATP). The specificity of the photolabeling was assured by first trapping [14C]Bz2ATP at the active site by use of thiol cross-linking agents [Mahmood, R., Cremo, C., Nakamaye, K., and Yount, R. (1987) J. Biol. Chem. 262, 14479-14486]. Five radioactive peptides were isolated by high-performance liquid chromatography after extensive trypsin and subtilisin digestion of photolabeled myosin subfragment 1. Four of these peptides were sequenced by Edman techniques, and all originated from a region with the sequence Gly-Glu-Ile-Thr-Val-Pro-Ser-Ile-Asp-Asp-Gln, which corresponds to rabbit myosin heavy chain residues 318-328. The fifth labeled peptide had an amino acid composition appropriate for residues 312-328. Amino acid composition, radiochemical analysis, and sequence data indicate that Ser-324 is the major amino acid residue photolabeled by Bz2ATP. Spectrophotometric evidence indicates that the benzophenone carbonyl group has inserted into a C-H bond from either the .alpha.- or .beta.-carbon of serine. These results place Ser-324 at a distance of 6-7 .ANG. from the 3''(2'') ribose oxygens of ATP bound at the active site of myosin.

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