FIGURE 7:

Model of Drosophila tropomyosin functions throughout the cell cycle. (A) During interphase, Tm1A colocalizes with myosin-II to the cell cortex and retracting membrane blebs, where they function to maintain cortical contractility and shape stability. In spreading cells, Tm1A and Tm1J localize with myosin-II to the lamellum and convergence zone, two networks previously shown to be important for cell motility (Salmon et al., 2002). Tm1J also colocalizes with Tm2A and Diaphanous to actin networks surrounding Golgi ministacks. There they help maintain Golgi architecture and, in so doing, influence cell cycle progression by preventing premature exit from G2 phase into mitosis. (B) During mitosis, Tm1A and myosin-II (Dean et al., 2005) initially colocalize to the equatorial cortex independent of Diaphanous but are maintained at the equator through a Diaphanous-dependent mechanism. Mislocalization of Tm1A causes shape instability and cytokinesis failure. Tm1J, on the other hand, localizes to the mitotic spindle. During metaphase, Tm1J localizes to kinetochores and centrosomes and may function with the troponin regulatory complex to influence spindle morphology, anaphase progression, and chromosome segregation. During anaphase, through a Diaphanous-dependent mechanism, Tm1J localization shifts to the central spindle and midbody, two structures previously shown to be important for cytokinesis (Giansanti et al., 1998).