The 'SUN' family: yeast SUN4/SCW3 is involved in cell septation

SUN4 is the fourth member of the SUN gene family from S. cerevisiae, whose products display high homology in their 258 amino acid C-terminal domain. S IM1, UTH1, NCA3 (the founding members) are involved in different cellular p rocesses (DNA replication, ageing, mitochondrial biogenesis) and it is show n herein that SUN4 plays a role in the cell septation process, sun4 Delta c ells are larger than wild-type and begin a new cell cycle before they have separated from their mother cell. This phenotype is more pronounced in sun4 Delta cells also deleted for UTH1. FAGS analysis shows apparent polyploidy which disappears when the cell cycle is arrested by mating factor or nocod azole, indicating that cell septation is delayed without modification of th e doubling time. Elutriated sun4 Delta uth1 Delta daughter cells are born l arger, and therefore enter S phase sooner than their wild-type counterpart. S phase duration, as well as timing of Clb2 degradation, is normal, but ce ll septation is delayed. Sun4p/Scw3p was recently described as a cell wall protein (Cappellaro et al., 1998) and, consistent with this notion, electro n micrographs of sun4 Delta cells show defects in the final steps of cell w all septation. Our data suggest that Sun4p and Uth1p might contribute to th e regulated process of cell wall morphogenesis and septation. Copyright (C) 2000 John Wiley & Sons, Ltd.