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The Conformation of Myosin Heads in Relaxed Skeletal Muscle: Implications for Myosin-Based Regulation.

Abstract：

:In isolated thick filaments from many types of muscle, the two head domains of each myosin molecule are folded back against the filament backbone in a conformation called the interacting heads motif (IHM) in which actin interaction is inhibited. This conformation is present in resting skeletal muscle, but it is not known how exit from the IHM state is achieved during muscle activation. Here, we investigated this by measuring the in situ conformation of the light chain domain of the myosin heads in relaxed demembranated fibers from rabbit psoas muscle using fluorescence polarization from bifunctional rhodamine probes at four sites on the C-terminal lobe of the myosin regulatory light chain (RLC). The order parameter 〈P2〉 describing probe orientation with respect to the filament axis had a roughly sigmoidal dependence on temperature in relaxing conditions, with a half-maximal change at ∼19°C. Either lattice compression by 5% dextran T500 or addition of 25 μM blebbistatin decreased the transition temperature to ∼14°C. Maximum entropy analysis revealed three preferred orientations of the myosin RLC region at 25°C and above, two with its long axis roughly parallel to the filament axis and one roughly perpendicular. The parallel orientations are similar to those of the so-called blocked and free heads in the IHM and are stabilized by either lattice compression or blebbistatin. In relaxed skeletal muscle at near-physiological temperature and myofilament lattice spacing, the majority of the myosin heads have their light chain domains in IHM-like conformations, with a minority in a distinct conformation with their RLC regions roughly perpendicular to the filament axis. None of these three orientation populations were present during active contraction. These results are consistent with a regulatory transition of the thick filament in skeletal muscle associated with a conformational equilibrium of the myosin heads.

journal_name

Biophys J

journal_title

Biophysical journal

authors

Fusi L,Huang Z,Irving M

doi

10.1016/j.bpj.2015.06.038

subject

Has Abstract

pub_date

2015-08-18 00:00:00

pages

783-92

issue

eissn

0006-3495

issn

1542-0086

pii

S0006-3495(15)00621-9

journal_volume

109

pub_type

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The Conformation of Myosin Heads in Relaxed Skeletal Muscle: Implications for Myosin-Based Regulation.