Tropomyosin-regulated actomyosin-based contractility in nonmuscles cells

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Our goal is the elucidation of mechanisms that govern tropomyosin-regulated actomyosin-based contractility in nonmuscle cells. Tropomyosin-binding to bare actin filaments has been described with the notion gestalt-binding. However, gestalt-binding is easily disrupted by other actin-binding proteins and fails to provide a satisfactory explanation for the role of low molecular weight isoforms of tropomyosin in the formation of functionally distinct filament populations, which display well-defined isoform-composition and function. Our work is based on a model where NM-2 isoforms are required in addition to cytosolic actin and Tm isoforms to establish minimal functional entities with clearly defined properties. Myosin mediates all stereospecific interactions in the resulting ATmM complex, thereby enhancing the specificity of interactions between the individual actin, myosin, and tropomyosin isoforms that coexist in the human cells. The cyclic ATP-dependent interaction of myosin with actin and the production of force and movement support the formation of structurally distinct filament populations. Moreover, we presume that myosin motor activity enhances the dynamic nature of the resulting filament populations, their ability to fulfill a specific functional role, and their independent regulation in different regions of the cell. The thousands of potential combinations of splice isoforms and mutants are a particular challenge in performing this project. Based on our preliminary work, it appears likely that studies with a relatively small number of isoform combinations are sufficient to make accurate predictions about the most common ATmM complexes found in human cells. The availability of the individual rate and equilibrium constants for prototypic cytoskeletal complexes will greatly advance the field, as current modeling approaches are rather inaccurately based on a small number of studies performed with skeletal muscle alpha-actin. Reconstitution and detailed structural characterization of prototypic ATmM complexes promises to reveal insights in the code that defines structure-function relationships in Tm-regulated actomyosin-based contraction, membrane shaping and remodeling, and the integration of cellular signaling events.