W0287
Towards the Structure Determination of a Modulated Protein
Crystal: The Semicrystalline State of Profilin:Actin. Gloria E.O.
Borgstahl1, Kartik Narayan2, Jeff Lovelace1,
Edward Snell3, Henry Bellamy4, Uno Lindberg5
and C.E. Schutt2, 1Eppley Inst., 987696 Nebraska Medical
Center, Omaha, NE 68198, 2Dept. of Chemistry, Princeton Univ.,
Princeton, NJ 08544, 3Marshall Space Flight Center, Huntsville, AL
35812, 4CAMD, Louisiana State Univ., Baton Rouge LA 70806,
5Dept. of Zoological Cell Biology, Stockholm Univ., Stockholm,
Sweden.
One of the remaining challenges to structural biology is the
solution of modulated structures. Modulation of the molecular structures within
the crystal can produce satellite reflections or a superlattice of reflections
in reciprocal space. If the macromoleculeís crystal lattice is composed
of physiologically relevant packing contacts, structural changes induced under
physiological conditions can cause distortion relevant to the function and
biophysical processes of the molecule making up the crystal. By careful
measurement of the distortion, and the corresponding three-dimensional structure
of the distorted molecule, we will visualize the motion and mechanism of the
biological macromolecule(s). We have measured the modulated diffraction pattern
produced by the semicrystalline state of profilin:actin crystals using highly
parallel and monochromatic synchrotron radiation coupled with fine phi slicing
for structure determination. These crystals present a unique opportunity to
address an important question in structural biology. The modulation is believed
to be due to the formation of actin helical filaments from the actin beta ribbon
upon the pH-induced dissociation of profilin. To date, the filamentous state of
actin has resisted crystallization and no detailed structures are available. The
semicrystalline state of profilin:actin crystals provides a unique opportunity
to understand the many conformational states of actin. This knowledge is
essential for understanding the dynamics underlying shape changes and motility
of eukaryotic cells. The structure of the semicrystalline state of
profilin:actin will challenge and validate current models of muscle contraction
and cell motility. The methodology and theory under development will be easily
extendable to other systems.