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* M.E. Müller Institute for Microscopy, Biozentrum, University of Basel, CH-4056 Basel, Switzerland; and The effect of the type of metal ion (i.e., Ca2+,
Mg2+, or none) bound to the high-affinity divalent cation binding site (HAS) of actin on filament assembly,
structure, and dynamics was investigated in the absence
and presence of the mushroom toxin phalloidin. In
agreement with earlier reports, we found the polymerization reaction of G-actin into F-actin filaments to be
tightly controlled by the type of divalent cation residing
in its HAS. Moreover, novel polymerization data are
presented indicating that LD, a dimer unproductive by
itself, does incorporate into growing F-actin filaments.
This observation suggests that during actin filament formation, in addition to the obligatory nucleation-
condensation pathway involving UD, a productive filament dimer, a facultative, LD-based pathway is implicated whose abundance strongly depends on the exact
polymerization conditions chosen. The "ragged" and "branched" filaments observed during the early stages
of assembly represent a hallmark of LD incorporation
and might be key to producing an actin meshwork capable of rapidly assembling and disassembling in highly
motile cells. Hence, LD incorporation into growing actin filaments might provide an additional level of regulation of actin cytoskeleton dynamics. Regarding the
structure and mechanical properties of the F-actin filament at steady state, no significant correlation with the
divalent cation residing in its HAS was found. However, compared to native filaments, phalloidin-stabilized filaments were stiffer and yielded subtle but significant structural changes. Together, our data indicate
that whereas the G-actin conformation is tightly controlled by the divalent cation in its HAS, the F-actin conformation appears more robust than this variation.
Hence, we conclude that the structure and dynamics of
the Mg-F-actin moiety within the thin filament are not
significantly modulated by the cyclic Ca2+ release as it
occurs in muscle contraction to regulate the actomyosin interaction via troponin.
School of Biological
Sciences, University of Auckland, Auckland, New Zealand
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