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Division of Cell Biology, Medical Research Council Laboratory of Molecular Biology, Cambridge, CB2 2QH, United Kingdom
SPA2 encodes a yeast protein that is one of
the first proteins to localize to sites of polarized growth,
such as the shmoo tip and the incipient bud. The dynamics and requirements for Spa2p localization in living cells are examined using Spa2p green fluorescent protein fusions. Spa2p localizes to one edge of unbudded cells and subsequently is observable 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 localization to the bud tip.
Strikingly, a small domain of Spa2p comprised of 150 amino acids is necessary and sufficient for localization
to sites of polarized growth. This localization domain
and the amino terminus of Spa2p are essential for its
function in mating. Searching the yeast genome database revealed a previously uncharacterized protein which we name, Sph1p (Spa2p homolog), with significant homology to the localization domain and amino
terminus of Spa2p. This protein also localizes to sites of
polarized growth in budding and mating cells. SPH1,
which is similar to SPA2, is required for bipolar budding and plays a role in shmoo formation. Overexpression of either Spa2p or Sph1p can block the localization
of either protein fused to green fluorescent protein,
suggesting that both Spa2p and Sph1p bind to and are
localized by the same component. The identification of
a 150-amino acid domain necessary 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.
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