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J. Cell Biol.,
Volume 142, Number 6, September 21, 1998 1595-1604
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710-3011
Structural maintenance of chromosomes
(SMC) proteins function in chromosome condensation
and several other aspects of DNA processing. They are
large proteins characterized by an NH2-terminal nucleotide triphosphate (NTP)-binding domain, two long
segments of coiled coil separated by a hinge, and a
COOH-terminal domain. Here, we have visualized by
EM the SMC protein from Bacillus subtilis (BsSMC)
and MukB from Escherichia coli, which we argue is a
divergent SMC protein. Both BsSMC and MukB show
two thin rods with globular domains at the ends emerging from the hinge. The hinge appears to be quite flexible: the arms can open up to 180°, separating the terminal domains by 100 nm, or close to near 0°, bringing the
terminal globular domains together.
A surprising observation is that the ~300-amino
acid-long coiled coils are in an antiparallel arrangement. Known coiled coils are almost all parallel, and
the longest antiparallel coiled coils known previously
are 35-45 amino acids long. This antiparallel arrangement produces a symmetrical molecule with both an
NH2- and a COOH-terminal domain at each end. The
SMC molecule therefore has two complete and identical functional domains at the ends of the long arms.
The bifunctional symmetry and a possible scissoring action at the hinge should provide unique biomechanical
properties to the SMC proteins.
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