title: Characterizations of cross-bridges in the presence of saturating concentrations of MgAMP-PNP in rabbit permeabilized psoas muscle.
abstract: Several earlier studies have led to different conclusions about the complex of myosin with MgAMP-PNP. It has been suggested that subfragment 1 of myosin (S1)-MgAMP-PNP forms an S1-MgADP-like state, an intermediate between the myosin S1-MgATP and myosin S1-MgADP states or a mixture of cross-bridge states. We suggest that the different states observed result from the failure to saturate S1 with MgAMP-PNP. At saturating MgAMP-PNP, the interaction of myosin S1 with actin is very similar to that which occurs in the presence of MgATP. 1) At 1 degrees C and 170 mM ionic strength the equatorial x-ray diffraction intensity ratio I11/I10 decreased with an increasing MgAMP-PNP concentration and leveled off by approximately 20 mM MgAMP-PNP. The resulting ratio was the same for MgATP-relaxed fibers. 2) The two dimensional x-ray diffraction patterns from MgATP-relaxed and MgAMP-PNP-relaxed bundles are similar. 3) The affinity of S1-MgAMP-PNP for the actin-tropomyosin-troponin complex in solution in the absence of free calcium is comparable with that of S1-MgATP. 4) In the presence of calcium, I11/I10 decreased toward the relaxed value with increasing MgAMP-PNP, signifying that the affinity between cross-bridge and actin is weakened by MgAMP-PNP. 5) The degree to which the equatorial intensity ratio decreases as the ionic strength increases is similar in MgAMP-PNP and MgATP. Therefore, results from both fiber and solution studies suggest that MgAMP-PNP acts as a non hydrolyzable MgATP analogue for myosin.
authors.affiliation: National Institute of Arthritis, Musculoskeletal, and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892-7182, USA.
authors.forename: S M
authors.initials: SM
authors.lastname: Frisbie
authors.forename: S
authors.initials: S
authors.lastname: Xu
authors.forename: J M
authors.initials: JM
authors.lastname: Chalovich
authors.forename: L C
authors.initials: LC
authors.lastname: Yu
mesh_terms: D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000266:Adenylyl Imidodiphosphate / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D008956:Models, Chemical; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D015879:Myosin Subfragments / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D009994:Osmolar Concentration; D011817:Rabbits; D014961:X-Ray Diffraction
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D000266:Adenylyl Imidodiphosphate; D000255:Adenosine Triphosphate; D009218:Myosins