Adapting to the pit

uring meiosis I, some mechanism must allow homologous chromosomes to separate while keeping sister chromatids paired until meiosis II. How does a cell make this distinction? On page 219, Rogers et al. propose that in C. elegans the aurora-B kinase AIR-2 is largely responsible for ensuring that cohesion between chromosomes breaks down at the proper place and time. The authors also identified additional components in what is likely to be a conserved pathway controlling chromosome cohesion. When AIR-2 activity is inhibited by RNAi, meiotic cells in the worm do not D separate homologous chromosomes or sister chromatids. In metaphase I in normal meiotic cells, AIR-2 localizes distal to chiasmata, corresponding to the last points of contact between homologous chromosomes. In metaphase II, AIR-2 localizes to the last points of contact between sister chromatids. AIR-2 phosphorylates the cohesin protein REC-8 at a specific site in vitro, and inhibition of the CeGLC-7 ␣ or-␤ phosphatases causes AIR-2 to localize nonspecifically along chromosomes. The authors suggest that CeGLC-7 ␣ / ␤ phosphatases restrict AIR-2 localization temporally and spatially on meiotic chromosomes. AIR-2 phosphorylates REC-8 in its vicinity, causing the cohesin to be degraded and releasing chromosomal cohesion only in the appropriate location. ᭿ Adapting to the pit ell surface receptors that are internalized generally interact with the endocytic machinery through adaptor proteins. On page 315, Howard et al. describe the first example of a protein involved in recognizing endocytic targeting signals in yeast. The work links together several earlier observations about yeast actin dynamics and endocytosis, and suggests that an analogous system may exist in mammalian cells. The authors found that a sequence containing the amino acid motif NPFX (1,2) D, previously characterized as an endocytic targeting signal in yeast, is sufficient to direct the uptake of a truncated cell surface receptor. A two-hybrid screen for NPFX (1,2) D-binding proteins yielded Sla1p, which is known to interact with the endocytic machinery and regulate actin dynamics. Disrupting Sla1p expression inhibited NPFX (1,2) D-mediated endocytosis. Combined with previous findings, the results imply that Sla1p is part of C For healthy eyes and bones: got Lrp? he Wnt family of secreted proteins controls several crucial developmental processes, some of which are apparently mediated by Wnt coreceptors from the LDL receptor-related protein (Lrp) family. Now, on page 303, Kato et al. report that the targeted disruption of Lrp5 in mice causes a phenotype virtually identical to that …