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Department of Molecular, Cellular, and Developmental Biology, University of Colorado-Boulder, Boulder,
Colorado 80309-0347
The MPS1 gene from Saccharomyces cerevisiae encodes an essential protein kinase required for
spindle pole body (SPB) duplication and for the mitotic
spindle assembly checkpoint. Cells with the mps1-1 mutation fail early in SPB duplication and proceed
through monopolar mitosis with lethal consequences.
We identified CDC37 as a multicopy suppressor of
mps1-1 temperature-sensitive growth. Suppression is
allele specific, and synthetic lethal interactions occur
between mps1 and cdc37 alleles. We examined the
cdc37-1 phenotype for defects related to the SPB cycle.
The cdc37-1 temperature-sensitive allele causes unbudded, G1 arrest at Start (Reed, S.I. 1980. Genetics. 95:
561-577). Reciprocal shifts demonstrate that cdc37-1
arrest is interdependent with 
-factor arrest but is not a
normal Start arrest. Although the cells are responsive
to -factor at the arrest, SPB duplication is uncoupled
from other aspects of G1 progression and proceeds past
the satellite-bearing SPB stage normally seen at Start.
Electron microscopy reveals side-by-side SPBs at
cdc37-1 arrest. The outer plaque of one SPB is missing
or reduced, while the other is normal. Using the mps2-1
mutation to distinguish between the SPBs, we find that
the outer plaque defect is specific to the new SPB. This
phenotype may arise in part from reduced Mps1p function: although Mps1p protein levels are unaffected by the cdc37-1 mutation, kinase activity is markedly reduced. These data demonstrate a requirement for
CDC37 in SPB duplication and suggest a role for this
gene in G1 control. CDC37 may provide a chaperone
function that promotes the activity of protein kinases.
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