TY - JOUR

T1 - Myosin step size. Estimation from slow sliding movement of actin over low densities of heavy meromyosin

AU - Uyeda, Taro Q.P.

AU - Kron, Stephen J.

AU - Spudich, James A.

N1 - Funding Information:
We thank Dr T. Yanagida for communicating unpnb-lished results, Dr Y. Toyoshima for her valuable advice, Dr H. M. Warrick for setting up a computer analysis system to measure the centroids of fluorescence images, and Dr M. A. Titus for her help in protein purifications. T.Q.P.U. is supported by a Japan Society for the Promotion of Science (JSPS) Fellowship for Research Abroad and S.J.K. is a trainee of the Medical Scientist Training Program. This work was supported by a JSPS grant to T.Q.P.U. and NIH grant GM33289 to J.A.S.

PY - 1990/8/5

Y1 - 1990/8/5

N2 - We have estimated the step size of the myosin cross-bridge (d, displacement of an actin filament per one ATP hydrolysis) in an in vitro motility assay system by measuring the velocity of slowly moving actin filaments over low densities of heavy meromyosin on a nitrocellulose surface. In previous studies, only filaments greater than a minimum length were observed to undergo continuous sliding movement. These filaments moved at the maximum speed (vo), while shorter filaments dissociated from the surface. We have now modified the assay system by including 0.8% methylcellulose in the ATP solution. Under these conditions, filaments shorter than the previous minimum length move, but significantly slower than vo, as they are propelled by a limited number of myosin heads. These data are consistent with a model that predicts that the sliding velocity (v) of slowly moving filaments is determined by the product of vo and the fraction of time when at least one myosin head is propelling the filament, that is, v = vo {1-(1- ts tc)N}, where ts is the time the head is strongly bound to actin, tc is the cycle time of ATP hydrolysis, and N is the average number of myosin heads that can interact with the filament. Using this equation, the optimum value of ts tc to fit the measured relationship between v and N was calculated to be 0.050. Assuming d = vots, the step size was then calculated to be between 10 nm and 28 nm per ATP hydrolyzed, the latter value representing the upper limit. This range is within that of geometric constraint for conformational change imposed by the size of the myosin head, and therefore is not inconsistent with the swinging cross-bridge model tightly coupled with ATP hydrolysis.

AB - We have estimated the step size of the myosin cross-bridge (d, displacement of an actin filament per one ATP hydrolysis) in an in vitro motility assay system by measuring the velocity of slowly moving actin filaments over low densities of heavy meromyosin on a nitrocellulose surface. In previous studies, only filaments greater than a minimum length were observed to undergo continuous sliding movement. These filaments moved at the maximum speed (vo), while shorter filaments dissociated from the surface. We have now modified the assay system by including 0.8% methylcellulose in the ATP solution. Under these conditions, filaments shorter than the previous minimum length move, but significantly slower than vo, as they are propelled by a limited number of myosin heads. These data are consistent with a model that predicts that the sliding velocity (v) of slowly moving filaments is determined by the product of vo and the fraction of time when at least one myosin head is propelling the filament, that is, v = vo {1-(1- ts tc)N}, where ts is the time the head is strongly bound to actin, tc is the cycle time of ATP hydrolysis, and N is the average number of myosin heads that can interact with the filament. Using this equation, the optimum value of ts tc to fit the measured relationship between v and N was calculated to be 0.050. Assuming d = vots, the step size was then calculated to be between 10 nm and 28 nm per ATP hydrolyzed, the latter value representing the upper limit. This range is within that of geometric constraint for conformational change imposed by the size of the myosin head, and therefore is not inconsistent with the swinging cross-bridge model tightly coupled with ATP hydrolysis.

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U2 - 10.1016/0022-2836(90)90287-V

DO - 10.1016/0022-2836(90)90287-V

M3 - Article

C2 - 2143785

AN - SCOPUS:0024991350

SN - 0022-2836

VL - 214

SP - 699

EP - 710

JO - Journal of Molecular Biology

JF - Journal of Molecular Biology

IS - 3

ER -