Smooth muscle myosin gene products include two isoforms, SMA and SMB, differing by a 7-residue peptide in loop 1 (i7) at the myosin active site where ATP is hydrolyzed. Using chicken isoforms, previous work indicated that the i7 deletion in SMA prolongs strong actin binding by inhibiting active site ingress and egress of nucleotide when compared to i7 inserted SMB. Additionally, i7 deletion inhibits Pi release associated with the switch 2 closed→open transition in actin-activated ATPase. Switch 2 is far from loop 1 indicating i7 deletion has an allosteric effect on Pi release. Chicken SMA and SMB have unknown and robust nucleotide-sensitive tryptophan (NST) fluorescence increments, respectively. Human SMA and SMB both lack NST increments while Pi release in Ca2+ATPase is not impacted by i7 deletion. The NST reports relay helix movement following conformation change in switch 2 but in the open→closed transition. The NST is common to all known myosin isoforms except human smooth muscle. Other independent works on human SMA and SMB motility indicate no functional effect of i7 deletion. Smooth muscle myosin is a stunning example of species-specific myosin structure/function divergence underscoring the danger in extrapolating disease-linked mutant effects on myosin across species. 1. Introduction Myosin is the chemomechanical energy transducer in muscle hydrolyzing ATP to power actin movement during contraction. It has a globular head domain called subfragment 1 (S1) and a tail domain that forms dimers. S1 contains an active site for ATP hydrolysis, an actin binding site, and a lever-arm whose rotary movement cyclically applies tension to cause contractility while myosin is strongly actin bound. Myosin energy transduction starts in the ATP binding site where peptides are linked to the 7-stranded -sheet sense position and coordination of the ATP -phosphate [1, 2]. ATP hydrolysis occurs rapidly, but products are not released until actin binds. The C-loop portion of the actin binding site senses actin contact that is transmitted to two strands in the 7-stranded -sheet, releasing product with the opening of switch 2 and translating the relay -helix [3]. Relay helix linear movement, sensed by the nucleotide sensitive tryptophan (NST) [4, 5], impinges on the converter changing linear force into torque to rotate the lever-arm. The lever-arm converts torque generated in the motor into the linear step-size displacement [6] and undergoes shear strain due to the resisting load [7]. Strain is shared among the lever-arm and the bound myosin light chains, essential (ELC)
References
[1]
S. Park, K. Ajtai, and T. P. Burghardt, “Mechanism for coupling free energy in ATPase to the myosin active site,” Biochemistry, vol. 36, no. 11, pp. 3368–3372, 1997.
[2]
K. Ajtai, F. Dai, S. Park et al., “Near UV circular dichroism from biomimetic model compounds define the coordination geometry of vanadate centers in MeVi- and MeADPVi-rabbit myosin subfragment 1 complexes in solution,” Biophysical Chemistry, vol. 71, no. 2-3, pp. 205–220, 1998.
[3]
K. Ajtai, M. F. Halstead, M. Nyitrai, A. R. Penheiter, Y. Zheng, and T. P. Burghardt, “The myosin C-loop is an allosteric actin contact sensor in actomyosin,” Biochemistry, vol. 48, no. 23, pp. 5263–5275, 2009.
[4]
S. Park and T. P. Burghardt, “Tyrosine mediated tryptophan ATP sensitivity in skeletal myosin,” Biochemistry, vol. 41, no. 5, pp. 1436–1444, 2002.
[5]
T. P. Burghardt, S. Park, W.-J. Dong, J. Xing, H. C. Cheung, and K. Ajtai, “Energy transduction optical sensor in skeletal myosin,” Biochemistry, vol. 42, no. 19, pp. 5877–5884, 2003.
[6]
Y. Wang, K. Ajtai, and T. P. Burghardt, “Qdot labeled actin super-resolution motility assay measures low duty cycle muscle myosin step-size,” Biochemistry, vol. 52, pp. 1611–1621, 2013.
[7]
T. P. Burghardt and L. A. Sikkink, “Regulatory light chain mutants linked to heart disease modify the cardiac myosin lever-arm,” Biochemistry, vol. 52, pp. 1249–1259, 2013.
[8]
M. F. Halstead, K. Ajtai, A. R. Penheiter et al., “An unusual transduction pathway in human tonic smooth muscle myosin,” Biophysical Journal, vol. 93, no. 10, pp. 3555–3566, 2007.
[9]
T. J. Eddinger and D. P. Meer, “Myosin II isoforms in smooth muscle: heterogeneity and function,” The American Journal of Physiology—Cell Physiology, vol. 293, no. 2, pp. C493–C508, 2007.
[10]
A. S. Rovner, Y. Freyzon, and K. M. Trybus, “An insert in the motor domain determines the functional properties of expressed smooth muscle myosin isoforms,” Journal of Muscle Research and Cell Motility, vol. 18, no. 1, pp. 103–110, 1997.
[11]
A.-M. Lauzon, M. J. Tyska, A. S. Rovner, Y. Freyzon, D. M. Warshaw, and K. M. Trybus, “A 7-amino-acid insert in the heavy chain nucleotide binding loop alters the kinetics of smooth muscle myosin in the laser trap,” Journal of Muscle Research and Cell Motility, vol. 19, no. 8, pp. 825–837, 1998.
[12]
R. G. Yount, D. Lawson, I. Rayment et al., “Is myosin a “back door” enzyme?” Biophysical Journal, vol. 68, 4 supplement, pp. 44S–49S, 1995.
[13]
M. Cecchini, Y. Alexeev, and M. Karplus, “Pi release from myosin: a simulation analysis of possible pathways,” Structure, vol. 18, no. 4, pp. 458–470, 2010.
[14]
C. M. Yengo, L. R. Chrin, A. S. Rovner, and C. L. Berger, “Tryptophan 512 is sensitive to conformational changes in the rigid relay loop of smooth muscle myosin during the MgATPase cycle,” Journal of Biological Chemistry, vol. 275, no. 33, pp. 25481–25487, 2000.
[15]
D. A. Winkelmann, L. Bourdieu, A. Ott, F. Kinose, and A. Libchaber, “Flexibility of myosin attachment to surfaces influences F-actin motion,” Biophysical Journal, vol. 68, no. 6, pp. 2444–2453, 1995.
[16]
R. W. Lymn and E. W. Taylor, “Transient state phosphate production in the hydrolysis of nucleoside triphosphates by myosin,” Biochemistry, vol. 9, no. 15, pp. 2975–2983, 1970.
[17]
Y. M. Peyser, M. Ben-Hur, M. M. Werber, and A. Muhlrad, “Effect of divalent cations on the formation and stability of myosin subfragment 1-ADP-phosphate analog complexes,” Biochemistry, vol. 35, no. 14, pp. 4409–4416, 1996.
[18]
S. Park and T. P. Burghardt, “Isolating and localizing ATP-sensitive tryptophan emission in skeletal myosin subfragment 1,” Biochemistry, vol. 39, no. 38, pp. 11732–11741, 2000.
[19]
K. A. Johnson and E. W. Taylor, “Intermediate states of subfragment 1 and actosubfragment 1 ATPase: reevaluation of the mechanism,” Biochemistry, vol. 17, no. 17, pp. 3432–3442, 1978.
[20]
R. Léguillette, F. R. Gil, N. Zitouni, S. Lajoie-Kadoch, A. Sobieszek, and A.-M. Lauzon, “(+)Insert smooth muscle myosin heavy chain (SM-B) isoform expression in human tissues,” The American Journal of Physiology—Cell Physiology, vol. 289, no. 5, pp. C1277–C1285, 2005.
[21]
P. Alhopuro, D. Phichith, S. Tuupanen et al., “Unregulated smooth-muscle myosin in human intestinal neoplasia,” Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 14, pp. 5513–5518, 2008.
[22]
S.-Q. Kuang, C. S. Kwartler, K. L. Byanova et al., “Rare, nonsynonymous variant in the smooth muscle-specific isoform of myosin heavy chain, MYH11, R247C, alters force generation in the aorta and phenotype of smooth muscle cells,” Circulation Research, vol. 110, no. 11, pp. 1411–1422, 2012.
[23]
J. R. Sellers, G. Cuda, F. Wang, and E. Homsher, “Myosin-specific adaptations of the motility assay,” Methods in Cell Biology, vol. 39, pp. 23–49, 1993.
