Structural dynamics of F-actin: II. Cooperativity in structural transitions.
A large body of biochemical evidence suggests that the F-actin filament can have internal cooperativity. We have observed large cooperative effects on the low-resolution structure of actin filaments under three very different conditions. First, when G-Ca(2+)-actin is polymerized by both Mg2+ and KCl, filaments may be found in two different populations, with two discrete positions seen for subdomain 2. When G-Ca2+ actin is polymerized by only Mg2+, a single F-Mg(2+)-actin population is seen. The structural data suggest that an entire filament exists with subdomain 2 in one state or the other when there is a heterogenous mixture of Mg2+ and Ca(2+)-actin. Second, when actin filaments are nucleated from gelsolin there is a conformational change that can be observed throughout the filament that is consistent with a large shift in the actin C terminus. There must be a large cooperative propagation of this effect throughout the filament from the nucleation point. Third, we have used phalloidin to stabilize F-actin in which two C-terminal residues have been proteolytically removed by trypsin. It has been shown biochemically that this stabilization occurs at substoichiometric amounts of phalloidin. Phalloidin, at either a 1:1 or a 1:20 molar ratio with actin, restores the connectivity between the long-pitch helical strands. F-actin's internal cooperativity will have large implications in vivo, particularly in muscle.
Department of Cell Biology and Neuroanatomy, University of Minnesota Medical School, Minneapolis 55455.
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Orlova
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Prochniewicz
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Egelman
D000199:Actins / Q000737:chemistry* / Q000648:ultrastructure; D000818:Animals; D018260:Gelsolin / Q000737:chemistry; D006868:Hydrolysis; D008854:Microscopy, Electron; D018482:Muscle, Skeletal / Q000737:chemistry; D010590:Phalloidine / Q000737:chemistry; D011487:Protein Conformation; D011817:Rabbits; D014357:Trypsin / Q000737:chemistry
D000199:Actins; D018260:Gelsolin; D010590:Phalloidine; D014357:Trypsin