SCF (Skp1-Cdc53-F-box protein) complexes
appear to be E3 ubiquitin protein ligases that target a number of
important regulatory proteins for ubiquitin dependent proteolysis. The
F-box proteins serve as exchangeable adaptor subunits that specifically
recruit various substrates to a core Cdc34-Cdc53-Skp1 E2/E3
ubiquitination complex. The SCF complex that contains the F-box protein
Cdc4 (SCF Cdc4 ) targets the Clb-Cdc28 inhibitor Sic1 for
degradation, whereas SCF Grr1 targets the G1 cyclin Cln2 for
degradation and SCF Met30 mediates repression of methionine
biosynthesis genes. SCF complexes appear to adhere to a generic
architecture in which Skp1 bridges the F-box protein to Cdc53, which in
turn binds Cdc34. Cdc53 contains non-overlapping binding sites for Cdc34
and Skp1 and does not contain a catalytically essential cysteine
residue. Cdc53 thus appears to function as a scaffold protein within the
E2/E3 core complex. While multiple substrates of SCF Cdc4 and
SCF Grr1 have been identified, the target of
SCF Met30 in repression of MET genes has not been
determined. Met30 is essential and physically interacts with the
MET specific transcriptional activator Met4. Deletion of
MET4 bypasses the lethality of the met30 deletion,
suggesting that one potential target of SCF Met30 may be Met4.
Progress on assembly of SCF Met30 , its potential targets, and
characterization of met30 temperature sensitive alleles will be
presented.
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