SPC110P - ASSEMBLY PROPERTIES AND ROLE IN THE CONNECTION OF NUCLEAR MICROTUBULES TO THE YEAST SPINDLE POLE BODY

Spc110p is an essential component of the budding yeast spindle pole bo dy (SPB). It binds calmodulin and contains a long central coiled-coil rod which acts as a spacer element between the central plaque of the S PB and the ends of the nuclear or spindle microtubules. This suggests that the essential function of Spc110p is to connect the nuclear micro tubules to the SPB. To confirm this, we examined the phenotype of ts a lleles of SPC110, one of which contains a mutation in the calmodulin b inding site and was suppressed by overexpression of calmodulin. The al leles fail to form a functional mitotic spindle because spindle microt ubules are not properly connected to the SPB. We also examined the phe notype of the toxic overexpression of either the wild-type or a trunca ted version of Spc110p containing a deletion of most of the coiled-coi l domain. Both of these proteins form large ordered spheroidal polymer s in the nucleus. The polymerization of the truncated Spc110p appears to be initiated inside the SPB from the position where Spc110p is norm ally located, and as the polymer grows in size it severs the connectio n between the nuclear microtubules and the SPB. The polymers were puri fied and are composed of Spc110p and calmodulin, A model for the struc ture of the polymer is proposed.