Electron microscopy performed by Melby et al. (page ) provides new clues as to how structural maintenance of chromosomes (SMC) proteins might help in two enormous tasks: chromosome condensation and sister chromatid cohesion.
The primary structure of the SMCs provides a problem. The two motifs that constitute the putative ATP-binding site are located at opposite ends of the molecule, separated by a long coiled coil, an intervening stretch, and another coiled coil. If the coiled coil is aligned, these motifs could come together in three-dimensional space in one of two ways: the intervening sequence could act as a hinge, allowing the two ends of the molecule to fold back onto each other, or the SMC dimer could form in an antiparallel fashion. Melby et al. find that both of these possibilities occur.
Functional studies of SMCs have focused largely on yeast and Xenopus proteins, but Melby et al. look...
