Tropomyosin-troponin regulation of actin does not involve subdomain 2 motions.
Dynamic properties of F-actin structure prompted suggestions (Squire, J. M., and Morris, E. P. (1998) FASEB J. 12, 761-771) that actin subdomain 2 movements play a role in thin-filament regulation. Using fluorescently labeled yeast actin mutants Q41C, Q41C/C374S, and D51C/C374S and azidonitrophenyl putrescine (ANP) Gln(41)-labeled alpha-actin, we monitored regulation-linked changes in subdomain 2. These actins had fully regulated acto-S1 ATPase activities, and emission spectra of regulated Q41C(AEDANS)/C374S and D51C(AEDANS)/C374S filaments did not reveal any calcium-dependent changes. Fluorescence energy transfer in these F-actins mostly occurred from Trp(340) and Trp(356) to 5-(2((acetyl)amino)ethyl)amino-naphthalene-1-sulfonate (AEDANS)-labeled Cys(41) or Cys(51) of adjacent same strand protomers. Our results show that fluorescence energy transfer between these residues is similar in the mostly blocked (-Ca(2+)) and closed (+Ca(2+)) states. Ca(2+) also had no effect on the excimer band in the pyrene-labeled Q41C-regulated actin, indicating virtually no change in the overlap of pyrenes on Cys(41) and Cys(374). ANP quenching of rhodamine phalloidin fluorescence showed that neither Ca(2+) nor S1 binding to regulated alpha-actin affects the phalloidin-probe distance. Taken together, our results indicate that transitions between the blocked, closed, and open regulatory states involve no significant subdomain 2 movements, and, since the cross-linked alpha-actin remains fully regulated, that subdomain 2 motions are not essential for actin regulation.
Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, California 90095, USA.
J H
JH
Gerson
E
E
Kim
A
A
Muhlrad
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E
Reisler
D000199:Actins / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D009154:Mutation; D011817:Rabbits; D012441:Saccharomyces cerevisiae; D013329:Structure-Activity Relationship; D014335:Tropomyosin / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism*
D000199:Actins; D014335:Tropomyosin; D014336:Troponin