Paper ID: 358
University of Heidelberg, Center for Molecular Biology (ZMBH) (Germany)
The spindle pole body (SPB) is the microtubule organizing centre of budding yeast Saccharomyces cerevisiae. Due to the closed mitosis, the SPB is embedded in the nuclear envelope throughout the cell cycle. This enables the SPB to organize nuclear and cytoplasmic microtubules with functions in chromsome segregation and spindle orientation, respectively. Like the mammalian centrosome, the SPB duplicates once per cell cycle using the mother structure as an initiation platform. SPB duplication is initiated in early G1 phase of the cell cycle by the formation of the satellite, a miniature version of the SPB, at the distal end of the bridge. The bridge is an extension of the central plaque and is layered on top of both sides of the nuclear envelope. The cytoplasmic side of the bridge consists of the interacting proteins Sfi1, yeast centrin Cdc31 and the tail-anchored Kar1 protein. At the G1/S phase the satellite grows in size and becomes inserted into the nuclear envelope. Genetic screes identified the SPB insertion network (SPIN) components Bbp1, Mps2, Ndc1 and Nbp1 as SPB-associated factors that are important for the nuclear envelope insertion of the new SPB. In this study we have uncoupled SPB growth from nuclear envelope insertion. This approach revealed several novel findings: 1) The mother SPB can fuse with the satellite if not separated by the bridge structure; 2) Mixing of the central Spc42 SPB layer occurs during SPB fusion in karyogamy; 3) The SPIN components encircle the newly embedded SPB in the nuclear envelope and anchor the SPB in the nuclear membrane; 4) A nuclear pore complex (NPC) becomes recruited to the inserting SPB in G1/S phase. This NPC is important for the insertion of the new SPB into the nuclear envelope. We will discuss models for the role of this NPC in SPB nuclear membrane insertion.