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© The Rockefeller University Press, 0021-9525/1998//711 $5.00
The Journal of Cell Biology, Volume 142, Number 3, , 1998 711-722

Regular Articles

Mlc1p Is a Light Chain for the Unconventional Myosin Myo2p in Saccharomyces cerevisiae



Richard C. Stevens and Trisha N. Davis

Department of Biochemistry, University of Washington, Seattle, Washington 98195-7350

In Saccharomyces cerevisiae, the unconventional myosin Myo2p is of fundamental importance in polarized growth. We explore the role of the neck region and its associated light chains in regulating Myo2p function. Surprisingly, we find that precise deletion of the six IQ sites in the neck region results in a myosin, Myo2-{Delta}6IQp, that can support the growth of a yeast strain at 90% the rate of a wild-type isogenic strain. We exploit this mutant in a characterization of the light chains of Myo2p. First, we demonstrate that the localization of calmodulin to sites of polarized growth largely depends on the IQ sites in the neck of Myo2p. Second, we demonstrate that a previously uncharacterized protein, Mlc1p, is a myosin light chain of Myo2p. MLC1 (YGL106w) is an essential gene that exhibits haploinsufficiency. Reduced levels of MYO2 overcome the haploinsufficiency of MLC1. The mutant MYO2-{Delta}6IQ is able to suppress haploinsufficiency but not deletion of MLC1. We used a modified gel overlay assay to demonstrate a direct interaction between Mlc1p and the neck of Myo2p. Overexpression of MYO2 is toxic, causing a severe decrease in growth rate. When MYO2 is overexpressed, Myo2p is fourfold less stable than in a wild-type strain. High copies of MLC1 completely overcome the growth defects and increase the stability of Myo2p. Our results suggest that Mlc1p is responsible for stabilizing this myosin by binding to the neck region.

Key Words: myosin • polarized • stability • Myo4 • cytokinesis

Abbreviations used in this paper: 5'-FOA, 5'-fluoro orotic acid; DAPI, 4',6-diamidino-2-phenylindole; GFP, green fluorescent protein; GST, glutathione-S-transferase.

Address all correspondence to Trisha N. Davis, Department of Biochemistry, Box 357350, University of Washington, Seattle, WA 98195-7350. Tel.: (206) 543-5345. Fax: (206) 685-1792. E-mail: tdavis{at}u.washington.edu


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