Meanwhile, back at the ring canal…

n page 703, Kelso et al. complete the trilogy of actin papers with a detailed biochemical analysis of the Drosophila Kelch protein. Like the Arp2/3 complex, Kelch is required for proper actin organization in ovarian ring canals. Although the structure of Kelch suggested that it might act as a dimeric actin cross-linking protein, this activity had not yet been demonstrated. In an impressive series of biochemical experiments, the authors demonstrate that purified Kelch can bundle actin filaments through a conserved actin-binding site, and that phosphorylation of a tyrosine residue near the actin-binding site blocks Kelch from interacting with actin. In vivo, Kelch is phosphorylated by a mechanism involving the Src-family kinase src64. A loss-of-function mutation in src64 and a mutation in Kelch that removes the phosphorylation site produce identical ring canal defects. The authors propose that ring canal growth is driven by actin polymerization and regulated actin cross-linking, in a mechanism similar to plasma membrane movement at the leading edge of motile cells. In this model, src64 phosphorylation of Kelch would be required to break cross-links, allowing rapid turnover of actin monomers. The similarity of src64 and Kelch mutant phenotypes also suggests that Kelch is the primary target of src64 activity during ring canal development. ᭿ O Actin (green) and Kelch help build ring canals. here are two models to explain how cells convert the physical forces of substrate adhesion into biochemical signals: force may open ion channels to induce localized changes in ion concentration across the plasma membrane, or physical distortion of the cytoskeleton may affect the signaling proteins associated with it. On page 609, Sawada and Sheetz provide significant new support for the second model. The authors grew cells on collagen-coated silicon, and then used detergent to strip the cells down to their cytoskeletons. When the cytoskeletons were stretched 10% and incubated with cytoplasmic proteins, the proteins bound at distinct spots. Relaxed cytoskeletons produced a different protein binding pattern. Biochemical analysis identified a distinct subset of proteins, including paxillin, focal adhesion kinase, and p130Cas, that bind in a stretch-dependent manner. Confirming the relevance of the system, the stretch-dependent binding pattern of GFP–paxillin appears identical in vitro and in vivo. The absence of a cell membrane in these experiments rules out the involvement of changes in ion concentration, and suggests that matrix forces directly cause conformational changes in the T Protein profiles differ in stretched versus relaxed cytoskeletons. ells …