Accumulation of some proteasomal substrates leads to slower cell growth. (A) Compromised degradation of Scc1, but not Arg-βgal, in ubr1Δ mutant cells causes toxicity. The plasmids bearing the N-end rule substrate Arg-βgal or Scc1 were separately introduced into wild-type JD52 or ubr1Δ cells as indicated. The expression of these substrates are regulated by the galactose-inducible GAL1 promoter. Exponentially growing yeast cells were spotted onto glucose-containing SD-ura (expression off) and galactose-containing SG-ura (expression on) media in serial five-fold dilutions. The plates were incubated at 30°C for two to four days. The substrates used are labeled on the left of the panel, and the strains are labeled on the right. (B) Increased levels of the UFD substrate UbV76-Val-βgal in rad23 dsk2 mutant cells do not affect cell growth. The spotting experiments were performed as in A.