Polymerization and in Vitro Motility Properties of Yeast Actin: A Comparison with Rabbit Skeletal α-Actin

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Abstract
Actin purified from the yeast (Saccharomyces cerevisae) was polymerized faster than rabbit skeletal α-actin by MgCl2. The two actins polymerized at similar rates in the presence of CaCl2. Yeast actin, up to 25 μM, was not polymerized by KCl (100−300 mM); the monovalent salt also inhibited the MgCl2-induced polymerization of actin. The local structure of the subdomain-2 region in yeast actin filaments was probed by subtilisin and trypsin digestions. Loop 38−52 appeared more flexible and accessible to subtilisin in yeast than in rabbit actin. In contrast, tryptic digestions at Lys-61 and -68 occurred at the same rate for yeast and α-actin filaments. Modification of yeast actin by a sulfhydryl reagent CPM [7-(diethylamino)-3-(4‘-maleimidophenyl)-4-methylcoumain] was specific to the Cys-374 residue; no labeling of a yeast actin mutant containing an alanine substitution for cysteine 374 was observed. The rates of Cys-374 labeling by CPM were similar for yeast and muscle actin, suggesting a similar environment for the C terminus in both polymers. In the in vitro motility assays, yeast actin required higher concentrations of heavy meromyosin (HMM) for its sliding than did the rabbit actin. At saturating concentrations of HMM, the sliding velocities of both actins were the same (3.0 μm/s). Relative forces generated by HMM with yeast and muscle actin were assessed by monitoring their in vitro motility in the presence of NEM−HMM load. The sliding of yeast actin was stopped at a level of external load (molar ratio NEM−HMM/HMM = 0.25) lower than that of muscle actin (NEM−HMM/HMM = 0.43), suggesting lower force production with yeast actin. These results are discussed in terms of the myosin cross-bridge cycle and actomyosin interactions.