Previous analysis of the bipolar budding pattern of Saccharomyces cerevisiae has suggested that it depends on persistent positional signals that mark the region of the division site and the tip of the distal pole on a newborn daughter cell, as well as each previous division site on a mother cell. In an attempt to identify genes encoding components of these signals or proteins involved in positioning or responding to them, we identified 11 mutants with defects in bipolar but not in axial budding. Five mutants displaying a bipolar budding-specific randomization of budding pattern had mutations in four previously known genes (BUD2, BUD5, SPA2, and BNI1) and one novel gene (BUD6), respectively. As Bud2p and Bud5p are known to be required for both the axial and bipolar budding patterns, the alleles identified here probably encode proteins that have lost their ability to interact with the bipolar positional signals but have retained their ability to interact with the distinct positional signal used in axial budding. The function of Spa2p is not known, but previous work has shown that its intracellular localization is similar to that postulated for the bipolar positional signals. BNI1 was originally identified on the basis of genetic interaction with CDC12, which encodes one of the neck-filament-associated septin proteins, suggesting that these proteins may be involved in positioning the bipolar signals. One mutant with a heterogeneous budding pattern defines a second novel gene (BUD7). Two mutants budding almost exclusively from the proximal pole carry mutations in a fourth novel gene (BUD9). A bud8 bud9 double mutant also buds almost exclusively from the proximal pole, suggesting that Bud9p is involved in positioning the proximal pole signal rather than being itself a component of this signal.

A search for Saccharomyces cerevisiae proteins that interact with actin in the two-hybrid system and a screen for mutants that affect the bipolar budding pattern identified the same gene, AIP3/BUD6. This gene is not essential for mitotic growth but is necessary for normal morphogenesis. MATa/alpha daughter cells lacking Aip3p place their first buds normally at their distal poles but choose random sites for budding in subsequent cell cycles. This suggests that actin and associated proteins are involved in placing the bipolar positional marker at the division site but not at the distal tip of the daughter cell. In addition, although aip3 mutant cells are not obviously defective in the initial polarization of the cytoskeleton at the time of bud emergence, they appear to lose cytoskeletal polarity as the bud enlarges, resulting in the formation of cells that are larger and rounder than normal. aip3 mutant cells also show inefficient nuclear migration and nuclear division, defects in the organization of the secretory system, and abnormal septation, all defects that presumably reflect the involvement of Aip3p in the organization and/or function of the actin cytoskeleton. The sequence of Aip3p is novel but contains a predicted coiled-coil domain near its C terminus that may mediate the observed homo-oligomerization of the protein. Aip3p shows a distinctive localization pattern that correlates well with its likely sites of action: it appears at the presumptive bud site prior to bud emergence, remains near the tips of small bund, and forms a ring (or pair of rings) in the mother-bud neck that is detectable early in the cell cycle but becomes more prominent prior to cytokinesis. Surprisingly, the localization of Aip3p does not appear to require either polarized actin or the septin proteins of the neck filaments.

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GTP binding proteins of the Sec4/Ypt/rab family regulate distinct vesicular traffic events in eukaryotic cells. We have cloned GDI1, an essential homolog of bovine rab GDI (GDP dissociation inhibitor) from the yeast Saccharomyces cerevisiae. Analogous to the bovine protein, purified Gdi1p slows the dissociation of GDP from Sec4p and releases the GDP‐bound form from yeast membranes. Depletion of Gdi1p in vivo leads to loss of the soluble pool of Sec4p and inhibition of protein transport at multiple stages of the secretory pathway. Complementation analysis indicates that GDI1 is allelic to sec19‐1. These results establish that Gdi1p plays an essential function in membrane traffic and are consistent with a role for Gdi1p in the recycling of proteins of the Sec4/Ypt/rab family from their target membranes back to their vesicular pools.

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