Published 18 February 2002. doi:10.1083/jcb1564iti5
© The Rockefeller University Press,
0021-9525/2002/2/587-b $5.00
The Journal of Cell Biology, Volume 156, Number 4, February 18, 2002 587-b-587
Meanwhile, back at the ring canal...
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Actin (green) and Kelch help build ring canals.
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On 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. 
Alan W. Dove
alanwdove{at}earthlink.net
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Drosophila Kelch regulates actin organization via Src64-dependent tyrosine phosphorylation
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