A unique loop contributing to the structure of the ATP-binding cleft of skeletal muscle myosin communicates with the actin-binding site.
Actin binding to skeletal muscle myosin subfragment-1 (S1) increases the dissociation rate of reaction products from the myosin ATPase site; conversely, ATP binding facilitates dissociation of complexed acto-S1. However, details of the molecular mechanism by which the ATP- and actin-binding sites communicate with each other is still obscure. We present evidence that the effect of actin is mediated by a conformational change in the loop containing amino acids from 677 to 689 [loop M (677-689)], a segment of the 20-kDa tryptic fragment that contributes to the structure of the ATP-binding cleft. Initially, a fluorescent ADP analogue, methylanthranyloyl-8-azido-ADP (Mant-8-N3-ADP), was covalently crosslinked to loop M (Mant-S1), perhaps at Lys 681. Actin-activated Mg2+-ATP hydrolysis by Mant-S1 was accelerated approximately 6 times over that by unmodified S1, suggesting that the ATPase site is not blocked by the ADP analogue crosslinked in the loop M (677-689). Nevertheless, analysis of Mant-group fluorescence polarization and acrylamide-induced quenching showed the crosslinked probe to be entrapped within the ATP-binding cleft at a location where Mant-group rotational mobility was hindered, and where it was relatively inaccessible to the solvent. Exposing Mant-S1 to Mg2+-ATP and/or actin elicited similar decreases in fluorescence polarization, indicating increased rotational mobility of the Mant-group and movement of crosslinked Mant-8-N3-ADP to a less hindered position. Stern-Volmer quench curves showed that Mant-8-N3-ADP was translocated to a site where it was more accessible to dissolved quencher, perhaps outside the ATP-binding cleft. Since actin does not bind to the ATPase site, actin-induced translocation of Mant-8-N3-ADP crosslinked to loop M (677-689) probably results from a conformational change in loop M (677-689). These results suggest that loop M acts as a signal transducer mediating communication between the ATP- and actin-binding sites.
Department of Bioengineering, Faculty of Engineering, Soka University, Hachioji, Tokyo, 192-8577, Japan. shinsaku@t.soka.ac.jp
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D000199:Actins / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002645:Chickens; D005454:Fluorescence Polarization; D018482:Muscle, Skeletal / Q000378:metabolism*; D009218:Myosins / Q000737:chemistry / Q000378:metabolism*; D010449:Peptide Mapping; D011487:Protein Conformation; D062367:ortho-Aminobenzoates / Q000378:metabolism
D000199:Actins; D062367:ortho-Aminobenzoates; C031385:anthranilic acid; D000255:Adenosine Triphosphate; D009218:Myosins