START transcriptional repressor; binds SBF (Swi4p-Swi6p) at SCB promoter elements in G1 to repress transcription; retains Cdc28-Cln3 at the ER and acts with Ydj1p to release Cln3p from the ER at critical size in a positive feedback loop to trigger START; unstable and degraded by SCF-Grr1 after hyperphosphorylation; contains a G1/S transcription factor binding (GTB) motif; binds SBF-regulated promoters in HU-treated cells; cell wall stress enhances repressor function; Cdc28p and Pho85p substrate