Sequences specify centromeric chromatin

n page 765, Ohzeki et al. present evidence for sequence specificity in assembling chromatin at mammalian centromeres. The sequence requirements imply that construction of centromeric chromatin is more than an epigenetic process. O Centromeric chromatin assembles at AT-rich repeats called ␣ satellite sequences, or alphoids. Type I alphoids contain boxes for the binding of CENP-B, one of several conserved protein components of centromeric chromatin. Alphoid sequences are also found in inactivated centromeres, and Y chromosomes form centromeric chromatin, although they lack CENP-B boxes. Thus, centromeres are thought to be inherited as a preassembled complex. However, assembly of new centromeric chromatin (de novo formation) requires specific sequences, according to the new results. The authors transformed cells with synthetic type I alphoid DNA constructs and looked for the formation of mammalian artificial chromosomes (MACs), which by definition have assembled functional centromeres de novo. Only alphoid constructs with CENP-B boxes elicited efficient MAC formation. Binding of CENP-B to its cognate DNA sequence initiated assembly of other centromere-specific proteins, including CENP-A,-C, and-E. CENP-B was not sufficient, however, as CENP-B boxes in a GC-rich context did not establish MAC formation. Thus, initial CENP-B binding is sequence specific, whereas further chromatin assembly appears to require the AT-rich repeats. This may reflect the preference of the histone-like CENP-A or other centromere components for AT-rich sequences during nucleosome folding. ᭿ MACs assemble chromatin proteins (green) on CENP-B–containing alphoid sequences (red). Microtubules get early release he centrosome is a nucleating center for microtubules T Nuclear travel costs energy he nucleus is a crowded place, with chromatin, nuclear speckles, and nucleoli clogging up the works. Although the usual crop of molecular motors have not been found in the nucleus, many nuclear molecules are able to move from place to place. Recently, diffusion has been argued to be the principal means of nuclear travel. But new results from Calapez et al. (page 795) argue that Brownian motion does not account for everything. Something is using energy to help large particles move in the nucleus. Large particles that must traverse the nucleus include mRNPs, a complex of mRNA and several associated protein factors. Calapez et al. analyzed mRNP movement from chromatin to the nuclear pore by FRAP analysis of two mRNA-binding proteins tagged with GFP. mRNP complexes moved more quickly than dextrans of the same size, which are expected to move by passive diffusion. So, mRNPs are getting help navigating the nucleus. …