A yeast gene, SPA2, was isolated with human anti-spindle pole autoantibodies. The SPA2 gene was fused to the Escherichia coli trpE gene, and polyclonal antibodies were prepared to the fusion protein. Immunofluorescence experiments indicate that the SPA2 gene product has a sharply polarized distribution in yeast cells. In budded cells the SPA2 protein is present at the tip of the bud; in unbudded cells, it is localized to one edge of the cell. When a-cells are induced to form schmoos with alpha-factor, the SPA2 protein is found at the tip of the schmoo. These areas of SPA2 localization correspond to cellular sites expected to be involved in bud formation and/or cell growth. The SPA2 antigen is present in a-cells, alpha-cells, and a/alpha-diploid cells, but is absent in mutant cells in which the SPA2 gene has been disrupted. spa2 mutant cells are viable, but display defects in the direction and control of cell growth. Compared to wild-type cells, spa2 mutant cells have slightly altered budding patterns. Entry into stationary phase is impaired for spa2 mutants, and mutants with one particular allele, spa2-7, form multiple buds under nutrient-limiting conditions. Thus, SPA2 is a newly identified yeast gene that is involved in the direction and control of cell division, and whose gene product localizes to the site of cell growth.

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