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Inovirus (filamentous bacteriophage) is a simple system for studying the rules
by which protein primary structure (amino acid sequence) controls secondary and
higher order structure, and thereby function. The virus occurs naturally as a
number of different strains with similar secondary and higher order structure,
but the protein subunit that assembles to form the virion coat has quite
different primary structures in different virus strains. Despite these
differences in primary structure, the subunits of all strains have much the same
size, about 50 residues, which are distributed by type in much the same way into
three domains of primary structure: a collection of acidic residues in the
N-terminal region, a hydrophobic domain of about 19 residues near the middle,
and a collection of basic residues near the C-terminus. Each subunit can be
closely approximated by an alpha-helix with its long axis roughly parallel to
the fibre axis, sloping from large to small radius in the virion and
interleaving between subunits in the next turn or level. The acidic residues
near the N-terminus of the subunit face outwards on the virion surface, and
explain the low isoelectric point of the virion; the basic residues near the
C-terminus face inwards, where they neutralize the charge on the DNA at the core
of the virion; and the hydrophobic central domain is involved in interactions
which bind neighbouring subunits. Detailed X-ray fibre diffraction analysis of
one strain gives the subunit structure. Comparative model-building studies of
different strains illustrate the common structural principles.
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