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MBC in Press, published online ahead of print February 2, 2005
Mol. Biol. Cell 10.1091/mbc.E04-08-0739

A more recent version of this article appeared on April 1, 2005
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Submitted on August 25, 2004
Revised on December 28, 2004
Accepted on January 17, 2005

Highly Cooperative Control of Endocytosis by Clathrin

Howard S. Moskowitz, Charles T. Yokoyama, and Timothy A. Ryan

Department of Biochemistry, Weill Medical College of Cornell University, New York, NY 10021

Monitoring Editor: Keith Mostov

Clathrin assembles into a dynamic two-dimensional lattice on the plasma membrane where it plays a critical role in endocytosis. To probe the regulation of this process, we used siRNA against clathrin, in combination with single cell assays for transferrin uptake as well as total internal reflection microscopy to examine how endocytic rates and membrane dynamics depend upon cellular clathrin concentration ([Clathrin]). We find that endocytosis is tightly controlled by [Clathrin] over a very narrow dynamic range such that small changes in [Clathrin] can lead to large changes in endocytic rates, indicative of a highly cooperative process (apparent Hill coefficient, n>6). The number of clathrin assemblies at the cell surface was invariant over a wide range of [Clathrin], however both the amount of clathrin in each assembly and the subsequent membrane dynamics were steeply dependent on [Clathrin]. Thus clathrin controls the structural dynamics of membrane internalization via a strongly cooperative process. We used this analysis to show that one important regulator of endocytosis, the actin cytoskeleton, acts noncompetitively as a modulator of clathrin function.

Address correspondence to: Timothy A. Ryan (taryan{at}med.cornell.edu)






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