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© The Rockefeller University Press, 0021-9525/1998//1595 $5.00
The Journal of Cell Biology, Volume 142, Number 6, , 1998 1595-1604

Regular Articles

The Symmetrical Structure of Structural Maintenance of Chromosomes (SMC) and MukB Proteins: Long, Antiparallel Coiled Coils, Folded at a Flexible Hinge



Thomas E. Melby, Charles N. Ciampaglio, Gina Briscoe, and Harold P. Erickson

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.

Key Words: SMC • MukB • coiled coil • electron microscopy • chromosome

Abbreviations used in this paper: aa, amino acid(s); BsSMC, Bacillus subtilis SMC protein; FN-MukBcoil, MukBcoil with the fibronectin cell adhesion segment fused to the NH2 terminus; MukBcoil, MukB with both NH2- and COOH-terminal domains truncated; NTP, nucleotide triphosphate; SMC, structural maintenance of chromosomes.

Address all correspondence to Harold P. Erickson, Department of Cell Biology, Duke University Medical Center, Durham, NC 27710-3011. Tel.: (919) 684-6385. Fax: (919) 684-3687. E-mail: H.Erickson{at}cellbio.duke.edu


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