Myosin thick filaments and subunit exchange: a stochastic simulation based on the kinetics of assembly.

Subunit exchange between groups of myosin filaments at equilibrium in a volume similar to a sarcomere is simulated using Monte Carlo (probabilistic) methods. Five published kinetic parameters (three rate constants and two cooperativity parameters) which govern the assembly of thick filaments from purified myosin at pH 8.0 are used for the computations. Filament length distributions equivalent to those measured experimentally in the electron microscope result. Distinctive patterns of exchange emerge because cooperativity in myosin assembly is not confined to nucleation but functions throughout growth. Fluctuations in filament size, first apparent in the millisecond time domain, mediate exchange which first occurs at the tips of the filaments and then gradually progresses inwards toward the central bare-zone. Exchange rates decreased by an approximate factor of 10 per decade of time: full exchange takes years, 50% takes 28 h, and 10% takes a brief 100 ms. These data represent the fastest possible rates of exchange because synthetic myosin filaments lack the overall stabilizing influence of the copolymerizing proteins of native filaments. Exchange at equilibrium is therefore too slow to explain, for example, the much faster rates recorded in vivo for the complete replacement of one myosin isoform by another. Facilitated exchange where partial or complete filament dissociation is followed by the introduction of new subunits during reassembly offers a means of accelerating exchange. In this context, it is shown that the requisite disassembly and reassembly of myosin thick filaments can be completed in a minimum of a few seconds.