THE BUD4 PROTEIN OF YEAST, REQUIRED FOR AXIAL BUDDING, IS LOCALIZED TO THE MOTHER BUD NECK IN A CELL CYCLE-DEPENDENT MANNER/

a and alpha cells of the yeast Saccharomyces cerevisiae exhibit an axi al budding pattern, whereas a/alpha diploid cells exhibit a bipolar pa ttern. Mutations in BUD3, BUD4, and AXL1 cause a and alpha cells to ex hibit the bipolar pattern, indicating that these genes are necessary t o specify the axial budding pattern (Chant, J., and I. Herskowitz. 199 1. Cell. 65:1203-1212; Fujita, A., C. Oka, Y. Arikawa, T. Katagi, A. T onouchi, S. Kuhara, and Y. Misumi. 1994. Nature (Lond.). 372:567-570). We cloned and sequenced BUD4, which codes for a large, novel protein (Bud4p) with a potential GTP-binding moth. Bud4p is expressed and loca lized to the mother/bud neck in all cell types. Most mitotic cells con tain two apparent rings of Bud4 immunoreactive staining, as observed f or Bud3p (Chant, J., M. Mischke, E. Mitchell, I. Herskowitz, and J. R. Pringle. 1995. J. Cell Biol. 129: 767-778). Early G1 cells contain a single ring of Bud4p immunoreactive staining, whereas cells at START a nd in S phase lack these rings. The level of Bud4p is also regulated i n a cell cycle-dependent manner. Bud4p is inefficiently localized in b ud3 mutants and after a temperature shift of a temperature-sensitive m utant, cdc12, defective in the neck filaments. These observations sugg est that Bud4p and Bud3p cooperate to recognize a spatial landmark (th e neck filaments) during mitosis and support the hypothesis that they subsequently become a landmark for establishing the axial budding patt ern in G1.