A Gamete-specific, Sex-limited Homeodomain Protein in Chlamydomonas

Abstract

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− 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.

Comparative genomics in Chlamydomonas and Plasmodium identifies an ancient nuclear envelope protein family essential for sexual reproduction in protists, fungi, plants, and vertebrates

“…Five genes known to be specific to plus gametes (the flagellar adhesion protein SAG1, the plus mating structure adhesion protein FUS1, the plus homeodomain protein GSP1, and two proteins of unknown function: MTA1 and MTA2) (Kurvari et al 1998;Ferris et al 2002;Misamore et al 2003) were found in three related clusters (C26, C36, and C39) ( Fig. 2B; Supplemental Table S3).…”

Flagellar Adhesion between mt+ and mt− Chlamydomonas Gametes Regulates Phosphorylation of the mt+-specific Homeodomain Protein GSP1

“…Affinity Purification of the Anti-GSP1 Antibody-A polyclonal antibody generated as described previously (22) was affinity-purified over a mixed-peptide column that contained both an N-terminal 16-amino acid peptide (PEATPSGQPPTHPHQQ) and a C-terminal 15-amino acid peptide (AEASTDHKRARTNNP) used in the generation of the antibody (22). For some experiments, the once-purified antibodies were further purified on a second column containing only the N-terminal peptide (22).…”

“…Recently, a screen in our laboratory carried out to identify molecules regulated by flagellar adhesion in Chlamydomonas (21) yielded a cDNA that encodes a homeodomain protein, GSP1, which is the first homeodomain protein to be identified in Chlamydomonas (22). Analysis by Southern blotting and immunoblotting indicated that although the gene for GSP1 is present in cells of both mating types (i.e.…”

Changes in Transcript Abundance in Chlamydomonas reinhardtii following Nitrogen Deprivation Predict Diversion of Metabolism

“…Because N deprivation triggers gametogenesis (Martin and Goodenough, 1975;Kurvari et al, 1998), we examined several genes known to be involved in mating-type plus (mt + ) gamete differentiation or sexual fusion in C. reinhardtii as internal controls for the induction state of the cells following N deprivation (Supplemental Table S1). Following N deprivation, cells had substantial increases in the abundance of transcripts of four of the six genes considered.…”