The mechanism of thin filament regulation: Models in conflict?

Abstract

In a recent JGP article, Heeley et al. (2019. J. Gen. Physiol https://doi.org/10.1085/jgp.201812198) reopened the debate about two- versus three-state models of thin filament regulation. The authors review their work, which measures the rate constant of P_i release from myosin.ADP.Pi activated by actin or thin filaments under a variety of conditions. They conclude that their data can be described by a two-state model and raise doubts about the generally accepted three-state model as originally formulated by McKillop and Geeves (1993. Biophys. J. https://doi.org/10.1016/S0006-3495(93)81110-X). However, in the following article, we follow Plato's dictum that "twice and thrice over, as they say, good it is to repeat and review what is good." We have therefore reviewed the evidence for the three- and two-state models and present our view that the evidence is overwhelmingly in favor of three structural states of the thin filament, which regulate access of myosin to its binding sites on actin and, hence, muscle contractility.

Figure 1.

Diagrammatic version of the three-state model as originally envisaged by McKillop and Geeves (1993). Tpm on a single strand of seven actin monomers can sit in one of three positions on the actin surface B, C, or O. In the B position, the major binding sites of myosin on actin are blocked by Tpm, and no significant binding of myosin is possible (weak electrostatic interaction may be possible). In the C position, myosin can bind to some of its binding site to form the relatively weakly Attached or A state, but rotation into the rigor-like state (R) is prevented by Tpm. More recent structural interpretations of the transition from A to R state would suggest that the A state is formed by the lower 50 kD domain of myosin binding to actin. The R state requires closure of the cleft between the upper and lower 50 kD domains (linked to switch one opening), allowing the upper 50 kD to access its binding site on actin. In the C state of the thin filament, the position of Tpm would sit between the upper and lower 50 kD domains forming a molecular gag preventing cleft closure. See Table 1 for the occupancy of the different states under different conditions. The term K_i refers to the equilibrium constants for each step of the scheme defined in the left to right or top down direction.

Figure 2.

Two-state model of the thin filament based on Scheme 2 of Heeley et al. (2019). In this model, the thin filament has two activity states, I and A. The linkage between structural transitions of thin filament complex and activity states are not detailed by Heeley et al. (2019). To avoid any assumptions about the structural transitions, the two activity states are shown as black and gray, respectively. The thin filament is predominantly in the I form in the absence of both calcium and myosin. The binding of either calcium (Ca²⁺) or a single strongly bound M will bias the system toward the A state, but neither is sufficient on its own to switch the system totally to the A form. See Table 2 for the fraction of the system on under different conditions.