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J. Cell Biol.,
Volume 144, Number 1, January 11, 1999 83-98


* Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; Coronin is a highly conserved actin-associated protein that until now has had unknown biochemical activities. Using microtubule affinity chromatography, we coisolated actin and a homologue of coronin,
Crn1p, from Saccharomyces cerevisiae cell extracts.
Crn1p is an abundant component of the cortical actin
cytoskeleton and binds to F-actin with high affinity (Kd
6 × 10
Swiss Institute for
Experimental Cancer Research, Lausanne, Switzerland; and § Department of Molecular Biotechnology, University of
Washington, Seattle, Washington 98195
9 M). Crn1p promotes the rapid barbed-end assembly of actin filaments and cross-links filaments into
bundles and more complex networks, but does not stabilize them. Genetic analyses with a crn1
deletion mutation also are consistent with Crn1p regulating filament assembly rather than stability. Filament cross-linking depends on the coiled coil domain of Crn1p,
suggesting a requirement for Crn1p dimerization. Assembly-promoting activity is independent of cross-linking and could be due to nucleation and/or accelerated
polymerization. Crn1p also binds to microtubules in
vitro, and microtubule binding is enhanced by the presence of actin filaments. Microtubule binding is mediated by a region of Crn1p that contains sequences (not found in other coronins) homologous to the microtubule binding region of MAP1B. These activities, considered with microtubule defects observed in crn1
cells and in cells overexpressing Crn1p, suggest that
Crn1p may provide a functional link between the actin and microtubule cytoskeletons in yeast.
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