TY - JOUR
T1 - Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments
AU - Lehman, William
AU - Hatch, Victoria
AU - Korman, Vicci
AU - Rosol, Michael
AU - Thomas, Lorie
AU - Maytum, Robin
AU - Geeves, Michael A.
AU - Van Eyk, Jennifer E.
AU - Tobacman, Larry S.
AU - Craig, Roger
N1 - Funding Information:
This study was funded by National Institutes of Health Research Grants HL36153 (to W. L.), AR34711 and HL62468 (to R. C.), HL38834 (to L. S. T.) and Shared Instrumentation Grant RR08426 (to R. C.) supporting electron microscope facilities. Additional funding was from a Wellcome Trust Program Grant 055841 (to M. G.) and a Medical Research Council of Canada Grant MT14375 (to J. E. V.). The ββ skeletal muscle tropomyosin used was prepared in the laboratory of Dr L. B. Smillie (University of Edmonton, by L. Thomas supported by an Alberta Heritage Postdoctoral Fellowship). We thank Drs C. Cohen (Brandeis University), S. S. Lehrer (Boston Biomedical Research Institute) and K. Allen (Boston University School of Medicine) for discussions during the course of these studies.
Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.
PY - 2000/9/22
Y1 - 2000/9/22
N2 - Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca2+ binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling. (C) 2000 Academic Press.
AB - Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca2+ binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling. (C) 2000 Academic Press.
KW - Actin
KW - Electron microscopy
KW - Muscle regulation
KW - Tropomyosin
KW - Troponin
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U2 - 10.1006/jmbi.2000.4080
DO - 10.1006/jmbi.2000.4080
M3 - Article
C2 - 10986121
AN - SCOPUS:0034703378
SN - 0022-2836
VL - 302
SP - 593
EP - 606
JO - Journal of molecular biology
JF - Journal of molecular biology
IS - 3
ER -