Hierarchical condensation

he movements of cilia and flagella are driven by axonemal dynein ATPases, whose activity must be coordinated in space and time. On page 47, Rupp and Porter provide the first molecular characterization of a component of the dynein regulatory complex (DRC), a crucial but poorly understood regulator of flagellar movement. Besides suggesting a model for DRC assembly, the work identifies a possible connection between motility regulation and a theoretical mechanism of cell growth arrest. Using insertional muta-genesis in Chlamydomonas , the authors identified a new mutation in the PF2 locus that causes motility defects. Further analysis showed that PF2 encodes subunit 4 of the DRC. The PF2 protein is uniformly distributed along the length of the axoneme and also associates with the basal body region, and its predicted structure has several coiled-coil domains that could mediate protein–protein interactions. PF2 mutants fail to assemble five of the seven known DRC subunits. The results suggest that PF2 acts as a molecular scaffold, stabilizing the DRC by interacting with other components of the complex. Close homologues of PF2 occur in a wide range of cell types, and include a trypanosome gene product required for directional motility, and mouse and human gene products enriched in growth-arrested cells. One exciting possibility is that primary cilia containing PF2 homologues may transmit a signal to the cell to initiate growth arrest. As a first step in confirming this idea, the localization of PF2-related products must be determined in cells lacking flagella. ᭿ T PF2 acts as a molecular scaffold in flagella. s cells progress from interphase through metaphase, their chromosomes undergo structural changes that are easy to observe but difficult to understand. Strukov et al. (page 23) attacked this problem by engineering chromosome regions that can be labeled selectively, allowing the authors to analyze chromosome condensation at high resolution. Their initial results provide strong support for one model of chromosome folding while contradicting predictions of another. Previous work has supported two different models of chromosome condensation. In the radial loop model, scaffold/ matrix-associated region (SAR/MAR) sequences in DNA anchor portions of the chromosome to a central scaffold, producing loops of 30-nm fibers. In the hierarchical folding model, however, there is a continuum of folding steps, independent of the SAR/MAR sequences, that produces more complicated structures than the 30-nm loops. In the new work, the authors created chromosome regions containing large copy numbers of a vector containing SAR/ …