Sticky polarity

Research Roundup Research Roundup for example, two nearby cells can somehow tell which one is closer to the equator (middle) of the eye, and specify the R3 (equatorial) and R4 (polar) fates, accordingly. " This is one of the great puzzles of PCP, " says Simon. " The cells are reading the gradient rather than the absolute values, and sensing the smallest differences. " For that process to have any fidelity, polarity researchers believe that communication between the neighboring cells is essential. " The process is not merely one of cells taking positional values but actively comparing positional values, " says Simon. " In that way, cadherins make sense, " because cell contact is probably needed for the comparison to take place. Furthermore, Simon thinks that differences in cadherin levels between the two cells could provide the information the cells need to sense direction. Yang and Simon began their investigation with a chance observation of eye polarity defects in a mutant defective for Fat, a cadherin superfamily member. Another Sticky polarity ell adhesion can instruct cell polarity, according to new results from Chung-They found that polarity across an epithelial surface—perpendicular to apical–basal polarity—involves interactions between cadherins. This form of planar cell polarity (PCP) achieves a remarkable feat. In the fly eye, an unusual function for nuclear actin. Rather than restricting itself to the cytoskeleton, actin may help anchor a chromatin-remodeling complex in the nucleus. Crabtree's study of antigen-stimulated lymphocyte activation has previously shown that phosphatidylinositol 4,5-bisphosphate (PIP 2) signaling helps retain a greater proportion of a mammalian chromatin remodeling complex called BAF in the nucleus. Rando and Crabtree have now determined that BAF binds PIP 2 directly, but only if the complex retains two particular subunits: actin and the actin-related protein BAF53. PIP 2 does not trigger exchange of BAF's actin subunit, but PIP 2 plus BAF does increase the extent of in vitro actin polymerization. In vitro, BAF binds PIP 2 vesicles and the ends and junctions of actin filaments. Crabtree found that actin contacted two separate domains of the Brg1 subunit of BAF, but only one of these contacts was regulated by PIP 2. He suggests that PIP 2 may free up a domain of actin so that it can interact with other actin subunits, thus anchoring the complex in the nucleus. The interaction is unlikely to involve long actin polymers. These have not been seen in the …