The UBL domain of Ssk1p is not required for the interaction with the proteasome. (A) UBL domain of Ssk1p. In the middle of the molecule (amino acids 243 to 290), Ssk1p contains a small domain homologous to ubiquitin and the UBL domains of human and yeast Rad23p. The UBL domain of Ssk1p shares amino acid identities of 42.6% with human Rad23p, 25.5% with yeast Rad23p, and 25.5% with ubiquitin. (B) Coimmunoprecipitation of Ssk1p with the proteasome. SSK1 (wild type [WT] or ΔUBL)-FLAG was expressed from pNS454 or pNS456, respectively, in the YPD1+ ssk1Δ hog1Δ (NS318) and ypd1Δ ssk1Δ hog1Δ (NS320) strains. Ssk1p (WT or ΔUBL)-FLAG was immunoprecipitated with anti-FLAG M2 beads, and proteasomes bound to Ssk1p were detected by Western blotting. +, present; Δ, deleted; −, not present.