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J. Cell Biol.,
Volume 142, Number 3, August 10, 1998 711-722
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-
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 MYO2-6IQ 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.
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