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Journal of Bacteriology, February 2001, p. 821-829, Vol. 183, No. 3
Department of Microbiology, North Carolina
State University, Raleigh, North Carolina 27695
Received 11 September 2000/Accepted 1 November 2000
A yeast glc7-1 mutant expressing a variant of protein
phosphatase type 1 fails to accumulate glycogen. This defect is
associated with hyperphosphorylated and inactive glycogen synthase,
consistent with Glc7p acting directly to dephosphorylate and activate
glycogen synthase. To characterize the glycogen synthesis defect of
this mutant in more detail, we isolated 26 pseudorevertants of the glc7-1 mutant. All pseudoreversion events were due to
missense mutations in GSY2, the gene encoding the major
isoform of glycogen synthase. A majority of the mutations responsible
for the suppression were in the 3' end of the gene, corresponding to
the phosphorylated COOH terminus of Gsy2p. Phosphorylation of the
mutant proteins was reduced, suggesting that they are poor substrates
for glycogen synthase kinases. Suppressor mutations outside this domain
did not decrease the phosphorylation of the resulting proteins,
indicating that these proteins are immune to the regulatory effects of
phosphorylation. Since no growth defect has been observed for strains
with altered glycogen levels, the relative levels of fitness of
GSY2 mutants that fail to accumulate glycogen and that
hyperaccumulate glycogen were assayed by cocultivation experiments. A
wild-type strain outcompeted both hypo- and hyperaccumulating strains,
suggesting that glycogen levels contribute substantially to the fitness
of yeast.
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.3.821-829.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Hyperactive Glycogen Synthase Mutants of
Saccharomyces cerevisiae Suppress the glc7-1
Protein Phosphatase Mutant
and
*
Corresponding author. Present address: Department of
Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130. Phone: (318)
675-7769. Fax: (318) 675-5180. E-mail: ktatch{at}lsuhsc.edu.
Present address: Department of Developmental Cell and Molecular
Biology, Duke University, Durham, NC 27708.
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