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J. Cell Biol.,
Volume 143, Number 7, December 28, 1998 1971-1980
* Department of Cell Biology and Neuroscience and During fertilization in Chlamydomonas, gametes of opposite mating types interact with each other
through sex-specific adhesion molecules on their flagellar surfaces. Flagellar adhesion brings the cell bodies of
the gametes into close contact and initiates a signal
transduction pathway in preparation for cell-cell
fusion. We have identified a cDNA, gsp1, whose transcript levels are upregulated during flagellar adhesion.
The GSP1 polypeptide is a novel, gamete-specific homeodomain protein, the first to be identified in an alga.
Its homeodomain shows significant identity with several higher plant homeodomain proteins. Although encoded by a single copy gene present in cells of both
mating types, immunoblot analysis showed that GSP1
was expressed in mating type (mt)+ gametes, but was
not detectable in mt Department of Pharmacology, The University of Texas Southwestern
Medical Center, Dallas, Texas 75235
gametes or in vegetative cells of
either mating type. Moreover, GSP1 appeared late during gametogenesis, suggesting that it may function during adhesion with mt gametes or after zygote formation. GSP1 is expressed in imp11, mt mutant gametes,
which have a lesion in the mid gene involved in sex determination and exhibit many phenotypic characteristics of mt+ gametes. Thus, gsp1 is negatively regulated
by mid and is the first molecule to be identified in Chlamydomonas that shows sex-limited expression.
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