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Vol. 14, Issue 6, 2543-2558, June 2003
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Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115
Submitted October 30, 2002;
Revised January 21, 2003;
Accepted February 5, 2003
Monitoring Editor: Howard Riezman
The Ste5 scaffold activates an associated mitogen-activated protein kinase
cascade by binding through its RING-H2 domain to a G
dimer
(Ste4/Ste18) at the plasma membrane in a recruitment event that requires prior
nuclear shuttling of Ste5. Genetic evidence suggests that Ste5 must
oligomerize to function, but its impact on Ste5 function and localization is
unknown. Herein, we show that oligomerization affects Ste5 activity and
localization. The majority of Ste5 is monomeric, suggesting that
oligomerization is tightly regulated. Increasing the pool of Ste5 oligomers
increases association with Ste11. Remarkably, Ste5 oligomers are also more
efficiently exported from the nucleus, retained in the cytoplasm by Ste11 and
better recruited to the plasma membrane, resulting in constitutive activation
of the mating mitogen-activated protein kinase cascade. Coprecipitation tests
show that the RING-H2 domain is the key determinant of oligomerization.
Mutational analysis suggests that the leucine-rich domain limits the
accessibility of the RING-H2 domain and inhibits export and recruitment in
addition to promoting Ste11 association and activation. Our results suggest
that the major form of Ste5 is an inactive monomer with an inaccessible
RING-H2 domain and Ste11 binding site, whereas the active form is an oligomer
that is more efficiently exported and recruited and has a more accessible
RING-H2 domain and Ste11 binding site.
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