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Department of Genetics, University of Washington, Seattle 98195
Proteasome-mediated protein degradation
is a key regulatory mechanism in a diversity of complex
processes, including the control of cell cycle progression. The selection of substrates for degradation clearly
depends on the specificity of ubiquitination mechanisms, but further regulation may occur within the proteasomal 19S cap complexes, which attach to the ends
of the 20S proteolytic core and are thought to control
entry of substrates into the core. We have characterized
a gene from Saccharomyces cerevisiae that displays extensive sequence similarity to members of a family of
ATPases that are components of the 19S complex, including human subunit p42 and S. cerevisiae SUG1/
CIM3 and CIM5 products. This gene, termed PCS1 (for
proteasomal cap subunit), is identical to the recently
described SUG2 gene (Russell, S.J., U.G. Sathyanarayana, and S.A. Johnston. 1996. J. Biol. Chem. 271:32810-
32817). We have shown that PCS1 function is essential
for viability. A temperature-sensitive pcs1 strain arrests
principally in the second cycle after transfer to the restrictive temperature, blocking as large-budded cells with a G2 content of unsegregated DNA. EM reveals
that each arrested pcs1 cell has failed to duplicate its
spindle pole body (SPB), which becomes enlarged as in
other monopolar mutants. Additionally, we have
shown localization of a functional Pcs1-green fluorescent protein fusion to the nucleus throughout the cell
cycle. We hypothesize that Pcs1p plays a role in the
degradation of certain potentially nuclear component(s) in a manner that specifically is required for SPB
duplication.
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