Ligase-dependent and independent functions of the C-terminus of Mms21 contribute to optimal growth and genome stability in Saccharomyces cerevisiae

An evolutionarily conserved E3 SUMO ligase, Mms21, orchestrates genome integrity processes. Our study examined a mutant of Saccharomyces cerevisiae Mms21, analogous to a mutant identified in a rare human condition characterized by genome instability. The human mutation C-terminally truncated the Mms21 protein, without affecting the residues in the E3 ligase domain. Thus, we hypothesized that the C-terminus regulated ligase-independent functions of Mms21. Truncating the last 22 amino acids of yeast Mms21 - designated as mms21Δ22 mutants - mimicked the human disease mutation. mms21Δ22 mutants exhibited slower growth and DNA damage sensitivity than the wild-type and two well-characterized mutants of Mms21 - one with two missense mutations in the enzymatic domain and another without the entire enzymatic domain and the C-terminus. Furthermore, mms21Δ22 mutants exhibited a G2/M delay during unchallenged growth. The mms21Δ22 allele reduced Mms21 protein levels, but the phenotypes of mms21Δ22 mutants simply could not be attributed to diminished protein levels. Our genetic data suggested that the C-terminus contributed to both ligase-dependent and -independent functions of Mms21 and opposed the activity of the adjacent domain, thereby fine-tuning genome integrity. The mms21Δ22 disease allele analog further enhanced our understanding of Mms21's functions beyond its ligase activity in genome instability conditions.