Sexual agglutinin of mating-type minus gametes in Chlamydomonas reinhardii. I. Loss and recovery of agglutinability of gametes treated with EDTA

The iso1 gene of Chlamydomonas is involved in sex determination

Sexual differentiation in the heterothallic alga Chlamydomonas reinhardtii is controlled by two mating-type loci, mt+ and mt-, which behave as a pair of alleles but contain different DNA sequences. A mutation in the mt minus-linked imp11 gene has been shown previously to convert a minus gamete into a pseudo-plus gamete that expresses all the plus gametic traits except the few encoded by the mt+ locus. Here we describe the iso1 mutation which is unlinked to the mt- locus but is expressed only in minus gametes (sex-limited expression). A population of minus gametes carrying the iso1 mutation behaves as a mixture of minus and pseudo-plus gametes: the gametes isoagglutinate but they do not fuse to form zygotes. Further analysis reveals that individual gametes express either plus or minus traits: a given cell displays one type of agglutinin (flagellar glycoprotein used for sexual adhesion) and one type of mating structure. The iso1 mutation identifies a gene unlinked to the mating-type locus that is involved in sex determination and the repression of plus-specific genes.

Signal transduction in the sexual life of Chlamydomonas

Several signal transduction pathways play important roles in the sexual life cycle of Chlamydomonas. Nitrogen deprivation, perhaps sensed as a drop in intracellular [NH4+], triggers a signal transduction pathway that results in altered gene expression and the induction of the gametogenic pathway. Blue light triggers a second signalling cascade which also culminates in gene induction and completion of gametogenesis. New screens have uncovered several mutants in these pathways, but so far we know little about the biochemical events that transduce the environmental signals of nitrogen deprivation and blue light into the changes in gene transcription that produce gametes. Cell-cell contact of mature, complementary gametes elicits a number of responses that prepare the cells for fusion. Contact is sensed by the agglutinin-mediated cross-linking of flagellar membrane proteins. An increase in [cAMP] couples protein cross-linking to the mating responses. In C. reinhardtii the cAMP signal appears to be generated by the sequential stimulation of as many as 3 distinct adenylyl cyclase activities. Although the molecular mechanisms of adenylyl cyclase activations are poorly understood, Ca2+ may play a role. Most of the mating responses appear to be triggered by a cAMP-dependent protein kinase, but here too, Ca2+ may play a role. Numerous mutants are facilitating studies of the signalling pathways that trigger the mating responses. Cell fusion triggers another series of events that culminate in the expression of zygote specific genes. The mature zygote is sensitive to a light signal which stimulates the expression of genes whose products are essential for germination. The signal transduction pathways that trigger zygospore formation and germination are ripe for investigation in this experimentally powerful system.

A monoclonal antibody that blocks adhesion of Chlamydomonas mt+ gametes

During the mating reaction (fertilization) in the biflagellated alga, Chlamydomonas reinhardtii, mt+ and mt- gametes adhere to each other via their flagella and subsequently fuse to form quadriflagellated zygotes. In the studies reported here, we describe a monoclonal antibody directed against an mt+ flagellar surface molecule. The antibody blocks the adhesiveness of mt+ gametes, isolated mt+ flagella, and detergent extracts thereof. It has no effect on mt- gametes. Cyanogen bromide-activated Sepharose beads derivatized with the antibody bind only mt+ gametes; mt- gametes and mt+ and mt- vegetative cells are unreactive with the derivatized beads. The interaction of mt+ gametes with the beads is dynamic and cells continuously bind, detach, and rebind to the beads. Surprisingly, antibody-derivatized beads that have been incubated with mt+ gametes acquire the ability to bind mt- gametes. Moreover, extraction of the preincubated beads with detergents releases active mt+ adhesion molecules. The evidence suggests that binding of the antibody to the flagellar surface adhesion molecules causes their release from the flagellar surface, possibly mimicking the normal mechanism of flagellar de-adhesion.

Light dependence of sexual agglutinability in Chlamydomonas eugametos

The mating activity of mating-type plus gametes of Chlamydomonas eugametos depends on light. Cells lost their ability to agglutinate with mating-type minus gametes after a dark period of 30 min. They regained their agglutinability after 10 min exposure to light. Other mating reactions, such as tipping and flagellar tip activation, were not dependent upon light. Since cycloheximide and tunicamycin did not affect the light-induced activation of flagellar agglutinability, no protein synthesis or glycosylation is involved in this process. Equal amounts of biologically active agglutination factor could be extracted from cells placed either in light or in darkness. A minor portion of the active material was found to be located on the flagellar surface of illuminated cells. No active material was found on the flagellar surface of dark-exposed cells, whereas their cell bodies contained the same amount of active material as the cell bodies of illuminated cells. Since a light-induced flow of agglutination factors from the cell body to the flagella could not be detected and dark-exposed cells could be slightly activated by amputation or fixation by glutaraldehyde, we propose that light affects flagellar agglutinability by an in-situ modification of the agglutination factor on the flagella. When mt (+) and mt (-) strains were crossed and the progeny examined for light-sensitivity, it was apparent that this phenomenon is not mating type-linked.

Characterization of the purified Chlamydomonas minus agglutinin

Chlamydomonas flagellar sexual agglutinins are responsible for the adhesion of opposite mating-type (plus and minus) gametes during the first stages of mating. Purification and partial characterization of the plus agglutinin was previously reported (Adair, W. S., C. J. Hwang, and U. W. Goodenough, 1983, Cell, 33:183-193). Here we characterize the purified minus molecule. We show it to be a high molecular weight, hydroxyproline-rich glycoprotein that migrates in the 3% stacking region of an SDS-polyacrylamide gel and is absent from two nonagglutinating minus mutants. Plus and minus agglutinins are remarkably similar, although nonidentical, in amino acid composition, molecular morphology, and reactivity in vivo and in vitro with monoclonal antibodies raised against the plus agglutinin. Moreover, the adhesiveness of both plus and minus agglutinins, when coupled to agarose beads, is abolished by thermolysin, trypsin, periodate, alkaline borohydride, reducing agents, or heat, but unaffected by exo- or endoglycosidases. The minus agglutinin, however, migrates just ahead of the plus molecule on SDS PAGE, is excluded from an anion-exchange (Mono Q) column, elutes earlier during hydrophobic interaction (Bio-gel TSK Phenyl 5PW) chromatography, and is sensitive to chymotrypsin digestion (unlike the plus agglutinin); therefore, it differs from the plus agglutinin in apparent molecular weight, net charge, relative hydrophobicity and proteolytic susceptibility. Nevertheless, our results generally demonstrate a high degree of homology between these complementary cell-cell recognition/adhesion molecules, which suggests that they are specified by genes that have a common evolutionary origin.