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Research has been conducted on condensin and cohesin, protein complexes mediating cohesion and condensation. Results suggest that the release of cohesin at the mitosis onset is important for sister chromatid resolution but is not important for condensin-mediated compaction.

Studies show that mitotic chromosome assembly is regulated by two specific classes of protein complexes within the structural maintenance of chromosomes (SMC) family. Cohesins bind to interphase chromatin separately from DNA replication, then dissociate once mitosis begins. Condensins are present in mitosis resulting from the cohesin binding, but are unaffected by cohesin immunodepletion.

An immunoaffinity approach utilizing antibodies against XCAP-C and XCAP-E was utilized for the purification and analysis of the chromosome condensation protein complexes. Analysis of the XCAP-C and XCAP-E multiprotein complexes indicated the presence of two major forms of condesins that were homologous to the Xenopus SMC family of proteins. Furthermore, the condesin complexes were characterized as components of the chromosome assembly machinery.

Research shows that the chromosomal proteins condensin I and II share the same pair of structural maintenance of chromosomes (SMC) subunits and in addition contain different sets of non-SMC subunits. Results demonstrate that they individually contribute to mitotic chromosome architecture and their absence causes severe defects in chromosome morphology.

Structural maintenance of chromosomes (SMC) proteins play central roles in regulating higher order chromosome dynamics from bacteria to humans. As judged by electron microscopy, the SMC homodimer from Bacillus subtilis (BsSMC) is composed of two antiparallel, coiled-coil arms with a flexible hinge. Site-directed cross-linking experiments show here that dimerization of BsSMC is mediated by a hinge-hinge interaction between self-folded monomers. This architecture is conserved in the eukaryotic SMC2-SMC4 heterodimer. Analysis of different deletion mutants of BsSMC unexpectedly reveals that the major DNA- binding activity does not reside in the catalytic ATPase domains located at the ends of a dimer. Instead, point mutations in the hinge domain that disturb dimerization of BsSMC drastically reduce its ability to interact with DNA. Proper hinge function is essential for BsSMC to recognize distinct DNA topology, and mutant proteins with altered hinge angles cross-link double-stranded DNA in a nucleotide-dependent manner. We propose that the hinge domain of SMC proteins is not a simple dimerization site, but rather it acts as an essential determinant of dynamic SMC-DNA interactions. [ABSTRACT FROM AUTHOR]

Structural maintenance of chromosomes (SMC) proteins play fundamental roles in higher-order chromosome dynamics from bacteria to humans. It has been proposed that the Bacillus subtilis SMC (BsSMC) homodimer is composed of two anti-parallel coiled-coil arms, each having an ATP-binding domain at its distal end. It remains totally unknown, however, how the two-armed structure supports ATP-dependent actions of BsSMC. By constructing a number of mutant derivatives including 'single-armed' BsSMC, we show here that the central hinge domain provides a structural flexibility that allows opening and closing of the two arms. This unique structure brings about bimodal regulation of the SMC ATPase cycle. Closing the arm can trigger ATP hydrolysis by allowing an end-end interaction within a dimer (intramolecular mode). When bound to DNA, ATP promotes a dimer-dimer interaction, which in turn activates their DNA-dependent ATPase activity (intermolecular mode). Our results reveal a novel mechanism of ATPase regulation and provide mechanistic insights into how eukaryotic SMC protein complexes could mediate diverse chromosomal functions, such as chromosome condensation and sister chromatid cohesion. [ABSTRACT FROM AUTHOR]

Studies of the mechanisms of osteoporosis have not yet determined whether these conditions result from increased osteoclast activity or decreased osteoblast activity, or perhaps both. Osteoporosis is related to aging and to postmenopausal status. The function and the mitotic capacity of cultured human osteoblast-like cells were investigated in this study. The age at which these cells lose the ability to divide showed a strong negative correlation with donor age (r = .815, p < .01). There was also significant correlation of maximum cell saturation density with donor age (r = .698, p < .01). Alkaline phosphatase (ALP) activity and mineralization ability, typical functions of osteoblasts, continue undiminished up to the point at which mitotic capacity ceases. When cells were treated with 1α,25-dihydroxy vitamin d3 in the presence of 2 mM α-glycerophosphate, ALP activity and mineralization ability showed an increase, rather than a decrease, with advancing donor age. However, these functions showed no correlation with in vitro cell aging. We propose that human osteoblast-like cells from elderly subjects do not lose their functions of mineralization and ALP formation, and that loss of these functions with cell aging does not correlate with advancing age. [ABSTRACT FROM PUBLISHER]