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J. Biol. Chem., Vol. 275, Issue 40, 30957-30961, October 6, 2000
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From the A gene, SIT4, was identified as
corresponding to a serine/threonine protein phosphatase and when
overexpressed confers lithium tolerance in galactose medium to the
budding yeast Saccharomyces cerevisiae. This gene has been
previously identified as a regulator of the cell cycle and involved in
nitrogen sensing. It is shown that the transcription levels of
SIT4 are induced by low concentrations of Li+
in a time-dependent manner. Na+ and
K+ at high concentrations, but not sorbitol, also induce
transcription. As a response to Na+ or Li+
stress, yeast cells lower the intracellular K+ content.
This effect is enhanced in cells overexpressing SIT4, which
also increase 86Rb efflux after the addition of
Na+ or Li+ to the extracellular medium. Another
feature of SIT4-overexpressing cells is that they maintain
a more alkaline pH of 6.64 compared with 6.17 in the wild type cells.
It has been proposed that the main pathway of salt tolerance in yeast
is mediated by a P-type ATPase, encoded by PMR2A/ENA1.
However, our results show that in a sit4 strain, expression
of ENA1 is still induced by monovalent cations, and
overexpression of SIT4 does not alter the amount of
ENA1 transcript. These results show that SIT4
acts in a parallel pathway not involving induction of transcription of
ENA1 and suggest a novel function for SIT4 in
response to salt stress.
Regulation of Monovalent Ion Homeostasis and pH by the Ser-Thr
Protein Phosphatase SIT4 in Saccharomyces
cerevisiae*
§,§
Departamento de Bioquímica
Médica, ICB-CCS, Universidade Federal do Rio de Janeiro, Rio de
Janeiro, R.J. 21941-590, Brazil and ¶ Departamento de
Genética Molecular, IFC, Universidad Nacional Autónoma de
México, Mexico City 04510, Mexico
*
This work was supported in part by grants from
Fundaçao de Amparo a Pesquisa do Rio de Janeiro (FAPERJ),
Conselho de Ciência e Tecnologia (CNPq), PADCT-Brazil (to
M. M.-L.), and Grant 400346-5 from Consejo Nacional de Ciencia y
Tecnología (Conacyt) México to Dr. Antonio Peña.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. de
Bioquímica Médica, ICB/CCS, Universidade Federal do Rio
de Janeiro, C.P. 68041, Rio de Janeiro, R.J. 21941-590, Brazil. Tel.:
55-021-590-4548; Fax: 55-021-270-8647; E-mail:
montero@server.bioqmed.ufrj.br.
Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.
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