![]() ![]() Immunoprecipitations from mitotic chromatin revealed that AKAP95 resides in a complex with hCAP-D2/Eg7, the human homologue of XCAP-D2/pEg7 ( Cubizolles et al. The former process is independent of A kinase activity, however, the latter requires cAMP signalling via A kinase. Recent antibody-blocking and rescue experiments have identified a role of the A kinase–anchoring protein, AKAP95, in chromatin condensation and maintenance of condensed chromosomes during mitosis and in mitotic extract ( Collas et al. However, the different behaviors of the two complexes during the cell cycle suggests that they may play distinct roles in chromosome architecture. What regulates the targeting of hCAP to chromosomes is unknown. The second complex (hSMC1/hSMC3) is required for metaphase progression ( Schmiesing et al. The human chromosome-associated protein (hCAP)-C/hCAP-E complex associates with chromosomes at mitosis and is required for chromosome condensation. Four SMC proteins have been identified in humans in the form of two distinct complexes ( Schmiesing et al. SMC homologues exist in a variety of organisms ranging from yeast to mammals ( Strunnikov and Jessberger 1999). ![]() 1998), additional processes are likely to regulate chromosomal targeting of condensins ( Hirano et al. However, as both interphase and mitotic forms of condensins bind DNA in a similar manner ( Kimura et al. Mitosis-specific phosphorylation of non-SMC proteins has been implicated in their targeting to chromosomes ( Hirano et al. Condensins are targeted to chromosomes at mitosis and in Xenopus egg extracts. The Xenopus 13S condensin complex consists of two SMC proteins (XCAP-C and -E) and three non-SMC elements (XCAP-D2/pEg7, -G, and -H Hirano et al. ![]() SMCs associate with non-SMC proteins in complexes, termed condensins, directly implicated in chromosome condensation ( Strunnikov and Jessberger 1999). Chromosome condensation requires a family of highly conserved ATPases called structural maintenance of chromosome (SMC) proteins ( Hirano and Mitchison 1994 Hirano et al. ![]() The eukaryotic genome must compact and resolve into distinct chromosomes for proper segregation at mitosis. The results implicate AKAP95 as a receptor that assists condensin targeting to chromosomes. GST pull-down data also suggest that AKAP95 recruits several condensin subunits. Eg7 and AKAP95 immunofluorescently colocalize to the central region of methanol-fixed metaphase chromosomes. Amount of Eg7 recruited correlates with extent of chromosome condensation: resolution into distinct chromosomes is obtained only when near-endogenous levels of Eg7 are recruited. Recombinant AKAP95 binds chromatin and elicits recruitment of Eg7 to chromosomes in a concentration-dependent manner. Condensation is rescued by a recombinant AKAP95 peptide containing the 306 COOH-terminal amino acids of AKAP95. When association of AKAP95 with chromatin is prevented, the chromatin does not condense. In HeLa cell mitotic extract, AKAP95 redistributes from the nuclear matrix to chromatin. We demonstrate here that AKAP95 acts as a targeting protein for human chromosome-associated protein (hCAP)-D2/Eg7, a component of the human condensin complex, to chromosomes. We previously showed that the nuclear A kinase–anchoring protein, AKAP95, is implicated in chromosome condensation. What regulates condensin targeting to chromatin is largely unknown. Association of the condensin multiprotein complex with chromatin is required for chromosome condensation at mitosis. ![]()
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