The SUN Protein Mps3 Is Required for Spindle Pole Body Insertion into the Nuclear Membrane and Nuclear Envelope Homeostasis

Abstract

The budding yeast spindle pole body (SPB) is anchored in the nuclear envelope so that it can simultaneously nucleate both nuclear and cytoplasmic microtubules. During SPB duplication, the newly formed SPB is inserted into the nuclear membrane. The mechanism of SPB insertion is poorly understood but likely involves the action of integral membrane proteins to mediate changes in the nuclear envelope itself, such as fusion of the inner and outer nuclear membranes. Analysis of the functional domains of the budding yeast SUN protein and SPB component Mps3 revealed that most regions are not essential for growth or SPB duplication under wild-type conditions. However, a novel dominant allele in the P-loop region, MPS3-G186K, displays defects in multiple steps in SPB duplication, including SPB insertion, indicating a previously unknown role for Mps3 in this step of SPB assembly. Characterization of the MPS3-G186K mutant by electron microscopy revealed severe over-proliferation of the inner nuclear membrane, which could be rescued by altering the characteristics of the nuclear envelope using both chemical and genetic methods. Lipid profiling revealed that cells lacking MPS3 contain abnormal amounts of certain types of polar and neutral lipids, and deletion or mutation of MPS3 can suppress growth defects associated with inhibition of sterol biosynthesis, suggesting that Mps3 directly affects lipid homeostasis. Therefore, we propose that Mps3 facilitates insertion of SPBs in the nuclear membrane by modulating nuclear envelope composition.

Author Summary

Accurate segregation of chromosomes during mitosis is essential to prevent genetic instability and aneuploidy that lead to cancer and other diseases. Centrosomes and spindle pole bodies mediate the assembly of a microtubule-based structure known as the mitotic spindle, which physically separates chromosomes during mitosis so that the two daughter cells contain a complete copy of the genetic material as well as a spindle pole. During every cell cycle, the DNA and the spindle pole must be duplicated exactly once to ensure proper formation of a bipolar mitotic spindle. In yeast cells, the nuclear envelope does not break down, so the spindle pole must be inserted into the nuclear membrane so that it can form both the microtubules involved in the mitotic spindle and those involved in positioning of the nucleus. How a large protein complex such as the spindle pole body is inserted into the lipid layers of the nuclear membrane is not well understood. We show that the evolutionarily conserved SUN protein Mps3 is involved in spindle pole insertion into the nuclear membrane. This likely reflects a function for SUN proteins in controlling nuclear envelope structure by modulating the types of lipids that are present in the nuclear membrane.