Wild-type (left), but not mutant (right), Mps1p can make an Spc42p super-plaque.

Castillo et al. (page 453) have identified what may be a key event in building the spindle pole body (SPB) of budding yeast. They found that the kinase Mps1p phosphorylates and thus helps organize Spc42p, an important component of the SPB central plaque.

Mps1p has been a difficult protein to study, as it does double duty as a regulator of SPB structure, and as part of the spindle checkpoint signal cascade. Previous conditional mutants have knocked out both functions, perhaps because the mutations have been focused in the catalytic domain of Mps1p. But Castillo et al. aim their mutagenesis at the essential NH2 terminus of the protein. They use a strain lacking Cin8p protein, as this condition makes the checkpoint essential, and thus ensures that any surviving mutant retains Mps1p checkpoint function.

The resulting SPB-assembly mutant, mps1–8, can be suppressed by overexpressed Spc42p. Spc42p coimmunoprecipitates with Mps1p, and is an in vitro substrate for its kinase activity. Finally, Mps1p is necessary for overexpressed Spc42p to form an extended super-plaque.

Mps1p is known to phosphorylate other SPB proteins at other times in SPB duplication. Thus Mps1p is one of several mitotic kinases that have multiple functions. The challenge now is to understand how these many functions are coordinated. ▪