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Biologische Systeme und Medizin Poster: Mi., 14:00–16:30 M-P181
Dynamic studies of an in situ molecular motor using synchrotron radiation
Christopher Ashley 1 , Maria Bagni 2 , Giovanni Cecchi 2 , Barbara Colombini 2 ,
Sergio Funari 3 , Peter Griffiths 1 , Radek Pelc 4
1 Univ. Lab. of Physiology, Parks Road, Oxford OX1 3PT, U.K. – 2 Dpt. di Scienze
Fisiol., Universita degli Studi di Firenze, Florence I-50134, Italy. – 3 Gebaeude 25F,
Hasylab at DESY, Notkestrasse 85, Hamburg D-22603, Germany – 4 Dept. of Microbiology,
Czech Academy of Sciences, 142 20 Prague 4-Krc, Czech Republic.
In higher animals, mechanical work is performed by translational molecular motors
(protein macromolecules which transduce the free energy of ATP hydrolysis into linear
motion). The most ubiquitous motor, myosin, achieves this by a multi-stage catalysis
of ATP hydrolysis, powering cardiac and smooth muscle, all voluntary movement,
and also adjustment of visual and aural acuity and myriad processes in intracellular
physiology. Myosins form a class of at least 17 different forms of translational motor
[1] Myosin II, present in all muscle tissue, is unique in that it forms helically-based
filamentous aggregates. In striated muscles, these are arranged in a quasi-crystalline
array, permitting structural events in the motor to be investigated dynamically by
time-resolved X-ray diffraction. The X-ray pattern from skeletal muscle contains a
strong axial reflection (M3) at a spacing corresponding to the third harmonic of the
pitch of the myosin helix, which is sensitive to changes in myosin molecular structure.
Upon activation, it undergoes changes in both intensity (IM3) and in axial spacing
(dM3) [2]. We have studied the changes in M3 which accompany the motor power
stroke in the microsecond time domain, and have attempted to relate these changes to
the crystallographic structure of the myosin molecule.
IM3 changes reflect structural events in the S1 moiety of myosin, which projects from
the surface of the myosin filament on a helical pitch and bears both the ATP catalytic
site and the binding sites for the filamentous protein, actin (the ’track’ along which
the myosin ’locomotive’ runs). Recent crystallographic work suggests that the motor
mechanism is a tilting of the ’lever arm’ domain of S1, an a-helix chain of 9nm in length,
linking the actin- and ATP-binding motor domain to the myosin filament backbone,
producing 2-5 pN of force if isometric, or ca. 10 nm of translation if movement is
permitted.
dM3 increases by ca. 1.5 % upon activation. This change is not explicable by structural
events in S1. Instead, it may indicate a change in the myosin filament backbone
structure, or the formation of a new axial unit cell due to the mismatch of actin and
myosin filament helices. Recently, we examined this phenomenon further [3]. Our
findings question the origin of this spacing change, suggesting that dM3 is not an
indicator of actin-S1 interaction, and allowing us to propose a new mechanism for dM3
changes which could have important consequences for the prevailing models of the
mechanism of contraction.
[1] Sellers, J.R. Biochim. Biophys. Acta. 1496 (2000) 3. [2] Huxley, H.E.et al., J. Mol.
Biol. 158 (1982) 63711. [3] Griffiths, P.J. et al., Biophys. J., 90 (2006) 975.