YBR267w designated
REI1 (required for isotropic bud growth) was isolated by two-hybrid screening using
NIS1 encoding the neck protein as bait. Disruption of
REI1 exhibited cold sensitive growth but did not exhibit a morphological defect. However, Δ
rei1Δ
nap1, Δ
rei1Δ
cla4 and Δ
rei1Δ
gin4 double disruptants exhibited an elongated cell morphology, which was suppressed by the disruption of
SWE1, indicating that
REI1 is a new member of genes belonging to the mitotic signaling network that negatively regulates Swe1 kinase. Δ
nap1 cells displayed a lower Gin4 kinase activity and a lower Gin4 protein level, both of which were recovered nearly to a wild type level in Δ
rei1Δ
nap1 cells. Interaction between Rei1 and Gin4 was suggested from our observation that Rei1 inhibited Gin4 kinase activity although weakly. The facts that although Δ
rei1Δ
nap1 cells displayed a severer elongated bud phenotype than Δ
nap1 cells, Gin4 kinase activity in Δ
rei1Δ
nap1 cells was higher than in Δ
nap1 cells, and that introduction of plasmid carrying a kinase inactive
gin4 mutant gene into Δ
rei1Δ
gin4 cells suppressed their morphological defect, indicate that kinase activity of Gin4 is not required for isotropic bud growth. We found that Rei1 is localized to the cytoplasm throughout the cell cycle. In view of the fact that members belonging to the mitotic signaling network are localized to the bud neck, at least at some stage of the cell cycle, Rei1 is a unique component of this pathway.
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