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All microfilaments are not created equal! Actin filaments are involved in many cellular activities at sites throughout the cell, including on the Golgi complex. This functional diversity involves differences in actin filament organization, dynamics, and mechanochemical properties, which are generated by molecules such as tropomyosin. The spatial segregation of tropomyosin isoforms is one mechanism suggested for regulating actin function at different sites (Bryce et al., [2002] Mol. Biol. Cell 14, 1002-1016). The cover images demonstrate different actin microfilaments, defined in part by isoforms of tropomyosin. Phalloidin-labeled stress fibers in the center panel represent one possible formation of microfilaments. In the lower panel, both stress fibers and shorter perinuclear microfilaments are labeled in green with an antibody that recognizes two tropomyosins, Tm5NM<-1 and Tm5NM-2. Red phalloidin staining colabels the stress fibers.

The top panel highlights the presence of customized microfilaments on the Golgi complex. Short microfilaments tightly associated with the perinuclear Golgi complex are stained in red by an antibody specific for the Tm5NM-2 isoform of tropomyosin, which is not on stress fibers (labeled with phalloidin in green). Percival et al., in the January issue of Molecular Biology of the Cell (vol. 15, pp 268-280, 2004), demonstrate Tm5NM-2 associating with short Golgi microfilaments and with vesicles for a possible role in vesicle budding. Images are by D. Brown from the same article.

-J.M. Percival, J.A.I. Hughes, D.L. Brown, G. Shevzov, K. Heimann, B. Vrhovski, N. Bryce, J.L. Stow, and P.W. Gunning