Cnm67p Is a Spacer Protein of the Saccharomyces cerevisiae Spindle Pole Body Outer Plaque

In Saccharomyces cerevisiae, the spindle pole body (SPB) is the functional homolog of the mammalian centrosome, responsible for the organization of the tubulin cytoskeleton. Cytoplasmic (astral) microtubules essential for the proper segregation of the nucleus into the daughter cell are attached at the outer plaque on the SPB cytoplasmic face. Previously, it has been shown that Cnm67p is an integral component of this structure; cells deleted for CNM67 are lacking the SPB outer plaque and thus experience severe nuclear migration defects. With the use of partial deletion mutants of CNM67, we show that the N- and C-terminal domains of the protein are important for nuclear migration. The C terminus, not the N terminus, is essential for Cnm67p localization to the SPB. On the other hand, only the N terminus is subject to protein phosphorylation of a yet unknown function. Electron microscopy of SPB serial thin sections reveals that deletion of the N- or C-terminal domains disturbs outer plaque formation, whereas mutations in the central coiled-coil domain of Cnm67p change the distance between the SPB core and the outer plaque. We conclude that Cnm67p is the protein that connects the outer plaque to the central plaque embedded in the nuclear envelope, adjusting the space between them by the length of its coiled-coil.