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VOLUME 83, NUMBER 25 PHYSICAL REVIEW LETTERS 20D ECEMBER 1999Comment on “Cooperative Molecular Motors”In a Letter published recently [1], Jülicher and Prostproposed a simple stochastical model for motor moleculesthat cooperate in large groups. In the case of muscle,it is well known that the interaction between actin and21myosin can lead to contraction if ATP and Ca arepresent [2]. A muscle fiber contains a large amount ofthese actin-myosin “motors,” and it thus seems natural toapply Jülicher and Prost’s model to the muscle fiber as away of studying some aspects of the fascinating behaviorof this system. The asymmetry of muscle fibers at thesarcomeric level has already been established, and one ofthe predictions of the stochastical model is that there is acritical point at which this system behaves as a liquid-vaporsystem; that is, velocity and external force are related by13jy2yj f 2 f at this point. The authors alsoc ext cpredict an instability of the system when ≠f ≠y , 0.ext FIG. 1. Qualitative comparison between the predictions of theThis instability is interpreted as a phase transition in which stochastical model (Ref. [1]) (solid line) and the experimentalthe system changes its direction of motion discontinuously results of Ref. [3] (hollow circles). The filled circle representsthe abnormal point at which the first and second derivatives ofwhen the external load exceeds the maximum load that they with respect to f are zero. The units of y ...