Gene activation by interaction of an inhibitor with a cytoplasmic
signaling protein.
Gang Peng, James Hopper
Biochemistry & Molecular Biol., Penn State Univ. College Med, 500 Univ.
Drive, Hershey, PA 17033, USA
GAL gene induction in yeast operates
through a galactose-dependent Gal3p-Gal80p interaction that somehow
overcomes Gal80p inhibition of Gal4p, the transcriptional activator. A
prevailing model specifies that addition of galactose does not cause
dissociation of the Gal80p-Gal4p complex, but converts the complex to an
active form. This non-dissociation model dictates that the inducer enter
the nucleus to activate the promoter-associated Gal4p-Gal80p complex.
Here we report that in response to galactose cytoplasmically confined
Gal3p can mediate induction and Gal80p dissociates from Gal4p. Gal3p was
targeted to plasma and vesicular membranes or the outer mitochondrial
membrane. These cytoplasmically tethered Gal3p derivatives were found to
function as well as wildtype Gal3p in providing galactose-responsive,
Gal4p-mediated GAL gene expression. To test for dissociation of Gal80p
from promoter-associated Gal4p we employed chromatin
immunoprecipitation. Antisera directed against Gal80p precipitated 3
fold less Gal4p-binding site DNA (UASgal) from galactose-induced cells
compared to non-induced cells. Thus, Gal80p occupancy of its binding
site(s) on promoter-associated Gal4p is reduced in response to galactose
addition. These results indicate that the Gal80p-Gal4p association is
more dynamic than previously envisioned. We propose that galactose-triggered Gal3p-Gal80p association in the cytoplasm activates Gal4p in
the nucleus through a linked-equilibrium mechanism.
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