Abstract #12

The Role of Mps2 and Mps3 at the Budding Yeast Spindle Pole Body Half-Bridge. Sue Jaspersen. Stowers Institute for Medical Research, Kansas City, MO.
   The budding yeast spindle pole body (SPB) nucleates both cytoplasmic and nuclear microtubules. The soluble core SPB is embedded in the nuclear envelope through interactions with at least one integral membrane protein, Mps2, and its binding partner Bbp1. Associated with one side of the core SPB is an electron-dense region of the nuclear envelope known as the half-bridge, which is important for SPB duplication as well as for microtubule nucleation during G1. Despite its importance in SPB function, the molecular details of half-bridge structure, including how the half-bridge is tethered to the core SPB, have not been well characterized. MPS3 encodes an essential integral membrane protein that localizes to the inner nuclear envelope face of the half-bridge. Sequence analysis showed that the C-terminus of Mps3 shares an approximately 150 amino acid region of homology with SUN domain proteins (for Sad1-UNC-84 homology), a family of integral inner nuclear envelope components that are thought to anchor other membrane proteins in the outer nuclear envelope to mediate attachment of the nucleus with the cytoskeleton. We found that the SUN domain in Mps3 interacts with the C-terminus of Mps2 to form a bridge between the inner and outer nuclear membranes. Analysis of mps2-381 and mps3-SUN mutants showed that Mps2-Mps3 binding is essential for SPB duplication, karyogamy and nuclear positioning. Based on the fact that the SPBs in mps2-381 and mps3-SUN mutants lack any recognizable half-bridge, we propose that Mps2-Mps3 binding is important to tether the membrane associated half-bridge with the soluble core SPB. Interestingly, we found that mps2-381 and mps3-SUN mutants also display defects in establishment of sister chromatid cohesion during S phase, chromosome positioning within the nucleus and transcriptional regulation. This suggests that the half-bridge may have nuclear functions distinct from the well characterized role of SPBs in microtubule nucleation and mitotic spindle formation.

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