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Published online 21 August 2000. doi:10.1083/jcb.150.4.895
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© The Rockefeller University Press, 0021-9525/2000/8/895/ $5.00
The Journal of Cell Biology, Volume 150, Number 4, August 21, 2000 895-904

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In Vivo Importance of Actin Nucleotide Exchange Catalyzed by Profilin

Amy K. Wolvena, Lisa D. Belmonta, Nicole M. Mahoneyb, Steven C. Almob, and David G. Drubina
a Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720-3202
b Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York, 10461

Correspondence to: David G. Drubin, Department of Molecular and Cell Biology, 401 Barker Hall, University of California, Berkeley, Berkeley, CA 94720-3202. Tel:(510) 642-3692 Fax:(510) 643-0062 E-mail:drubin{at}uclink4.berkeley.edu.

The actin monomer-binding protein, profilin, influences the dynamics of actin filaments in vitro by suppressing nucleation, enhancing nucleotide exchange on actin, and promoting barbed-end assembly. Profilin may also link signaling pathways to actin cytoskeleton organization by binding to the phosphoinositide PIP2 and to polyproline stretches on several proteins. Although activities of profilin have been studied extensively in vitro, the significance of each of these activities in vivo needs to be tested. To study profilin function, we extensively mutagenized the Saccharomyces cerevisiae profilin gene (PFY1) and examined the consequences of specific point mutations on growth and actin organization. The actin-binding region of profilin was shown to be critical in vivo. act1-157, an actin mutant with an increased intrinsic rate of nucleotide exchange, suppressed defects in actin organization, cell growth, and fluid-phase endocytosis of pfy1-4, a profilin mutant defective in actin binding. In reactions containing actin, profilin, and cofilin, profilin was required for fast rates of actin filament turnover. However, Act1-157p circumvented the requirement for profilin. Based on the results of these studies, we conclude that in living cells profilin promotes rapid actin dynamics by regenerating ATP actin from ADP actin–cofilin generated during filament disassembly.

Key Words: yeast, cytoskeleton, cofilin, filament turnover, ATP


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