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Holmes proposed that in F-actin, hydrophobic residues in a subdomain 3/4 loop interact with a hydrophobic pocket on the opposing strand resulting in helix stabilization. We have determined how a decreased hydrophobicity of this plug affects yeast actin function. Cells harboring only the V266G, V266D, V266F, L267G, L269D, or L269K actins appear normal, although V266G cells display an altered budding pattern. However, V266G,L267G (GG) double mutant cells are cold-sensitive with randomly oriented thick actin assemblies seen in rhodamine phalloidin-stained GG cells. V266D actin polymerizes slower than wild-type actin at room temperature. At 4°C, not only is polymerization slowed, but there is also an effect on critical concentration. However, the polymerization defects are milder than those associated with substitution of Asp for the neighboring Leu 267. Purified GG-actin does not polymerize in vitro alone or in the presence of wildtype F-actin seeds. GG-actin polymerization can be restored by larger amounts of wild-type actin, beryllium fluoride, or phalloidin at room temperature, although at 4°C only phalloidin is effective. These results suggest that the diminished hydrophobicity of the plug in GG-actin leads to filament destabilization. However, the V266D actin results require a modification of the original Holmes filament model.

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