Probable E3 ubiquitin-protein ligase which is a component of the spindle assembly checkpoint, required to prevent septum formation and premature exit from mitosis if spindle function is compromised. Inhibits the septation initiation netwok (SIN) during spindle checkpoint activation. The effect appears to be mediated through preventing the SIN activator, plo1 kinase, from localizing to the SPB. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Interacts with sid4. Localizes also at the division site in mitosis. The FHA domain is required for the proper localization at the SPB. Belongs to the DMA1 family.