A single amino acid change in the multikinase scaffold Ste5 uncouples
the efficiency and specificity of MAP kinase signal
transduction.
Monica A. Schwartz, Hiten D. Madhani
Department of Biochemistry, U.of California, San Francisco, 513
Parnassus, San Francisco, CA 94143-0448, USA
The mechanism by which
specificity is maintained is a central issue in signal transduction. The
mating and filamentation pathways of S. cerevisiae are distinct
developmental processes that respond to different cues, yet share
multiple MAPK cascade components. The mating-specific multikinase
scaffold Ste5 has been proposed to be required for the maintenance of
specificity, yet compelling evidence has been elusive. We have
identified mutations in the STE5 gene that produce inappropriate
cross-talk, in which the pheromone signal activates a filamentation
pathway-specific reporter. Remarkably, of four mutants that exhibited
cross-talk, all have changes in a single amino acid: E756. ste5-E756G causes cross-talk in a reporter assay and is hyperinvasive.
Importantly, this cross-talk mutant normally induces the pheromone
reporter FUS1-lacZ, and therefore uncouples signaling efficiency
from specificity. Cross-talk cannot be simply explained as a loss of the
activation of the Fus3 MAPK (which is required for signaling
specificity), since ste5-E756G does not show the synthetic
sterility with kss1 delta seen in a fus3 kss1 double
mutant. Moreover, E756 lies in the Ste7 (MAPKK) binding domain rather
than in the Fus3 interaction domain. A nearby mutation, D746G, that
reduces the Ste5-Ste7 interaction does not produce cross-talk. These
data suggest a novel specificity role for the Ste5 scaffold that is
distinct from its passive role as a kinase tether.
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