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2014 ; 552-553
(ä): 68-73
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The structural dynamics of ?-tropomyosin on F-actin shape the overlap complex
between adjacent tropomyosin molecules
#MMPMID24071513
Lehman W
; Li XE
; Orzechowski M
; Fischer S
Arch Biochem Biophys
2014[Jun]; 552-553
(ä): 68-73
PMID24071513
show ga
Coiled-coil tropomyosin, localized on actin filaments in virtually all eukaryotic
cells, serves as a gatekeeper regulating access of the motor protein myosin and
other actin-binding proteins onto the thin filament surface. Tropomyosin's
modular pseudo-repeating pattern of approximately 39 amino acid residues is
designed to allow binding of the coiled-coil to successive actin subunits along
thin filaments. Even though different tropomyosin isoforms contain varying
numbers of repeat modules, the pseudo-repeat length, in all cases, matches that
of a single actin subunit. Thus, the seven pseudo-repeats of 42nm long muscle
tropomyosin bind to seven successive actin subunits along thin filaments, while
simultaneously bending into a super-helical conformation that is preshaped to the
actin filament helix. In order to form a continuous cable on thin filaments that
is free of gaps, adjacent tropomyosin molecules polymerize head-to-tail by means
of a short (?9 residue) overlap. Several laboratories have engineered peptides to
mimic the N- and C-terminal tropomyosin association and to characterize the
overlap structure. All overlapping domains examined show a compact N-terminal
coiled-coil inserting into a partially opened C-terminal partner, where the
opposing coiled-coils at the overlap junction face each other at up to ?90° twist
angles. Here, Molecular Dynamics (MD) simulations were carried out to determine
constraints on the formation of the tropomyosin overlap complex and to assess the
amount of twisting exhibited by full-length tropomyosin when bound to actin. With
the exception of the last 20-40 C- and N-terminal residues, we find that the
average tropomyosin structure closely resembles a "canonical" model proposed in
the classic work of McLachlan and Stewart, displaying perfectly symmetrical
supercoil geometry matching the F-actin helix with an integral number of
coiled-coil turns, a coiled-coil helical pitch of 137Å, a superhelical pitch of
770Å, and no localized pseudo-rotation. Over the middle 70% of tropomyosin, the
average twisting of the coiled-coil deviates only by 10° from the canonical model
and the torsional freedom is very small (std. dev. of 7°). This small degree of
twisting cannot yield the orthogonal N- and C-termini configuration observed
experimentally. In marked contrast, considerable coiled-coil unfolding, splaying
and twisting at N- and C-terminal ends is observed, providing the conformational
plasticity needed for head-to-tail nexus formation.