A SMALL CONSERVED DOMAIN IN THE YEAST SPA2P IS NECESSARY AND SUFFICIENT FOR ITS POLARIZED LOCALIZATION

SPA2 encodes a yeast protein that is one of the first proteins to loca lize to sites of polarized growth, such as the shmoo tip and the incip ient bud. The dynamics and requirements for Spa2p localization in livi ng cells are examined using Spa2p green fluorescent protein fusions. S pa2p localizes to one edge of unbudded cells and subsequently is obser vable in the bud tip. Finally, during cytokinesis Spa2p is present as a ring at the mother-daughter bud neck. The bud emergence mutants bem1 and bem2 and mutants defective in the septins do not affect Spa2p loc alization to the bud tip. Strikingly, a small domain of Spa2p comprise d of 150 amino acids is necessary and sufficient for localization to s ites of polarized growth. This localization domain and the amino termi nus of Spa2p are essential for its function in mating. Searching the y east genome database revealed a previously uncharacterized protein whi ch we name, Sph1p (Spa2p homolog), with significant homology to the lo calization domain and amino terminus of Spa2p. This protein also local izes to sites of polarized growth in budding and mating cells. SPH1, w hich is similar to SPA2, is required for bipolar budding and plays a r ole in shmoo formation. Overexpression of either Spa2p or Sph1p can bl ock the localization of either protein fused to green fluorescent prot ein, suggesting that both Spa2p and Sph1p bind to and are localized by the same component. The identification of a 150-amino acid domain nec essary and sufficient for localization of Spa2p to sites of polarized growth and the existence of this domain in another yeast protein Sph1p suggest that the early localization of these proteins may be mediated by a receptor that recognizes this small domain.