Manuel F. Morales

Early stages of energy transduction by myosin: Roles of Arg in Switch I, of Glu in Switch II, and of the salt-bridge between them

On the basis of the crystallographic snapshots of Rayment and his collaborators [Fisher, A. J., Smith, C. A., Thoden, J. B., Smith, R., Sutoh, K., Holden, H. M., & Rayment, I. (1995) Biochemistry 34, 8960–8972], we have understood some basic principles about the early stages of myosin catalysis, namely, ATP is drawn into the active site, over which the cleft closes. Catalyzed hydrolysis occurs, and the first product (orthophosphate) is released from the backdoor of the cleft. In the cleft-closing process, the active site incidentally signals its movement to a particular remote tryptophan residue, Trp-512. In this work, we expand on some of these ideas to rationalize the behavior of a mutated system in action. From the behavior of recombinant myosin systems in which Arg-247 and Glu-470 were substituted in several ways, we draw the conclusions that (i) the force between Arg-247 and γ-phosphate of ATP may assist in closing the cleft, and incidentally in signaling to the remote Trp, and (ii) in catalysis, Glu-470 is involved in holding the lytic H2O (w1). We also propose that w1 and also a second water, w2, enter into a structure that bridges Glu-470 and the γ-phosphate of bound ATP, and at the same time positions w1 for its in-line hydrolytic attack.

Smooth Muscle Myosin

From their crystallographic comparisons, Fisher et al. (Biochemistry 34, 8960-8972, 1995) have proposed that in an important transition of myosin heads (M), MATP → M ADP Pi, an interdomain rotation occurs in Gly468 (of chicken smooth muscle myosin) and that the rotated state is stabilized by newly-formed interdomain contacts including the salt link between Glu470 and Arg247 (of chicken smooth muscle myosin). Here, we have studied the effects of Gly468, Glu470, and Arg247 mutations on the hydrolysis of ATP. The G468A HMM did not show a significant ATPase activity, a stoichiometric initial phosphate burst, and tryptophan fluorescence enhancement attributed to bound ADP Pi. The E470A HMM also did not show a significant ATPase activity and the phosphate burst, but the mutant gave tryptophan response attributed to bound ATP. The E470R/R247E HMM exhibited an ATPase activity and the phosphate burst which were comparable to those of the wild-type HMM, whereas neither the E470R HMM nor the R247E HMM showed such a significant ATPase activity and burst. We thus propose that both an unhindered rotation and a salt link that stabilizes the rotated state are necessary for ATP hydrolysis.

Changes in the 31P NMR spectrum of rabbit muscle myosin subfragment 1. MgADP with temperature.

In pioneering studies on the 31P NMR spectra of MgADP bound to the molecular motor myosin subfragment 1 (S1) in the temperature range of 0 to 25 degrees C, Shriver and Sykes [Biochemistry 20 (1981) 2004-2012/6357-6362; Biochemistry 21 (1982) 3022-3028], proposed that MgADP binds to myosin S1 as a mixture of two interconvertible conformers with different chemical shifts for the beta-P resonance of the S1-bound MgADP and that the concentrations of these conformers are related by an equilibrium constant K(T). Their model implied that the weighted average of the chemical shifts of the beta-P(MgADP) for S1-bound MgADP asymptotically approaches a high temperature limit. Here, and in our earlier paper [K. Konno, K. Ue, M. Khoroshev, H., Martinez, B.D. Ray, M.F. Morales, Proc. Natl. Acad. Sci. USA 97 (2000) 1461-1466], we report experimental similarities to Shriver and Sykes, but diverge from them (especially at 0 degrees C) in not finding two distinct peaks and in finding that the average chemical shift does not change with temperature. Our observations can be explained by chemical exchange of beta-P(MgADP) of S1-bound MgADP between two nearly energetically equivalent environments.

A Hypothesis About Myosin Catalysis

When ATP binds to the active site of myosin heads, Switch II undergoes a large conformational change and the cleft surrounding the bound gamma-phosphate closes. In the closed state, Glu470 in Switch II comes together with Arg247 in Switch I to form a salt-bridge. Here, the functional significance of the two bridging residues was tested by using site-directed mutagenesis. We conclude from such tests that (a) the attractive force between Arg247 and the gamma-phosphate of ATP moves the cleft to close, and (b) during hydrolysis, Glu470 is intimately involved in positioning the lytic water for the attack on the gamma-phosphorus. We also speculate on how the salt-bridge between Arg247 and Glu470 is related to hydrolysis.

The Region in Myosin S-1 that may be Involved in Energy Transduction

Newly-reported structural information about certain proximities between points on bound nucleotide and points on the heavy chain of myosin S-1 are incorporated into a previously-reported [Botts, J. Thomason, J.F. & Morales, M.F. Proc. Nat. Acad. Sci. USA, 86, 2204-2208 (1989)] structure of S-1. The resulting, enhanced structure is then used to identify some functionalities (e.g., the ATP-perturbable tryptophans), and to explain certain observations (e.g., some concerning the role of bound Mg2+ in the spectral response of TNBS-labelled Lys-83, and some concerning the response of the S-1 CD signal to nucleotide binding and to temperature change). These considerations lead to the suggestion that a strand of the 50 kDa domain (residues 510 to 540), and a strand of the 20 kDa domain (residues 697-719) are involved in transmitting the effects of nucleotide binding and hydrolysis to the loop (constituted from the same domain) that reaches a major (S-1)-actin interface.