Microtubule organization by the budding yeast spindle pole body

In budding yeast microtubule organizing functions are provided by the spind le pole body (SPB), a multi-layered structure that is embedded in the nucle ar envelope throughout the cell cycle. The SPB organizes the nuclear and cy toplasmic microtubules which are spatially and functionally distinct. Micro tubule formation in yeast requires the Tub4p-complex, containing the gamma- tubulin Tub4p, and two additional proteins, the SPB components Spc97p and S pc98p. The Tub4p complex assembles in the cytoplasm and is then anchored to the sides of the SPB which organize microtubules. This is achieved by the binding of Spc97p and Spc98p to so-called gamma-tubulin complex binding pro teins (GTBPs) at the SPB. Spc72p is the yeast GTBP at the cytoplasmic side of the SPB, while Spc110p is the nuclear GTBP. Both GTBPs control the numbe r of Tub4p complexes associated with the SPB and thereby the number of micr otubules formed. In addition, the GTBPs may regulate the activity of the Tu b4p complex. Homologues of Spc97p and Spc98p have been identified from yeas t to mammalian cells and these are also part of gamma-tubulin complexes, su ggesting that these related proteins may also interact with GTBPs at the ce ntrosome. Candidates for GTBPs have been identified in mammalian and insect cells. (C) 1999 Editions scientifiques et medicales Elsevier SAS.