Flagellar adhesion-dependent regulation of Chlamydomonas adenylyl cyclase in vitro: a possible role for protein kinases in sexual signaling

A cytoplasmic protein kinase couples engagement of Chlamydomonas ciliary receptors to cAMP-dependent cellular responses

The primary cilium is a cellular compartment specialized for receipt of extracellular signals essential for development and homeostasis. Although intraciliary responses to engagement of ciliary receptors are well studied, fundamental questions remain about the mechanisms and molecules that transduce ciliary signals into responses in the cytoplasm. During fertilization in the bi-ciliated alga Chlamydomonas reinhardtii, ciliary adhesion between plus and minus gametes triggers an immediate ∼10-fold increase in cellular cAMP and consequent responses in the cytoplasm required for cell-cell fusion. Here, we identify a new participant in ciliary signaling, Gamete-Specific Protein Kinase (GSPK). GSPK is essential for the adhesion-induced cAMP increase and for rapid gamete fusion. The protein is in the cytoplasm and the entire cellular complement responds to a signal from the cilium by becoming phosphorylated within 1 minute after ciliary receptor engagement. Unlike all other cytoplasmic events in ciliary signaling, GSPK phosphorylation is not responsive to exogenously added cAMP. Thus, during ciliary signaling in Chlamydomonas, a cytoplasmic protein is required to rapidly interpret a still uncharacterized ciliary signal to generate a cytoplasmic response.

Transient Internalization and Microtubule-Dependent Trafficking of a Ciliary Signaling Receptor from the Plasma Membrane to the Cilium

Cilia are ancient organelles used by unicellular and multicellular organisms not only for motility but also to receive and respond to multiple environmental cues, including light, odorants, morphogens, growth factors, and contact with cilia of other cells. Much is known about the cellular mechanisms that deliver membrane proteins to cilia during ciliogenesis. Execution of a ciliary signaling pathway, however, can critically depend on rapid alterations in the receptor composition of the cilium itself, and our understanding of the mechanisms that underlie these rapid, regulated alterations remains limited [1-6]. In the bi-ciliated, unicellular alga Chlamydomonas reinhardtii, interactions between cilia of mating type plus and mating type minus gametes mediated by adhesion receptors SAG1 and SAD1 activate a ciliary signaling pathway [7]. In response, a large, inactive pool of SAG1 on the plasma membrane of plus gametes rapidly becomes enriched in the peri-ciliary membrane and enters the cilia to become active and maintain and enhance ciliary adhesion and signaling [8-14]. Ciliary entry per se of SAG1 is independent of anterograde intraflagellar transport (IFT) [13], but the rapid apical enrichment requires cytoplasmic microtubules and the retrograde IFT motor, dynein 1b [14]. Whether the receptors move laterally within the plasma membrane or transit internally during redistribution is unknown. Here, in coupled immunolocalization/biochemical studies on SAG1, we show that, within minutes after gamete activation is initiated, cell-surface SAG1 is internalized, associates with an apico-basally polarized array of cytoplasmic microtubules, and returns to the cell surface at a peri-ciliary staging area for entry into cilia.

Sexual reproduction and sex determination in green algae

The sexual reproductive processes of some representative freshwater green algae are reviewed. Chlamydomonas reinhardtii is a unicellular volvocine alga having two mating types: mating type plus (mt⁺) and mating type minus (mt^-), which are controlled by a single, complex mating-type locus. Sexual adhesion between the gametes is mediated by sex-specific agglutinin molecules on their flagellar membranes. Cell fusion is initiated by an adhesive interaction between the mt⁺ and mt^- mating structures, followed by localized membrane fusion. The loci of sex-limited genes and the conformation of sex-determining regions have been rearranged during the evolution of volvocine algae; however, the essential function of the sex-determining genes of the isogamous unicellular Chlamydomonas reinhardtii is conserved in the multicellular oogamous Volvox carteri. The sexual reproduction of the unicellular charophycean alga, Closterium peracerosum-strigosum-littorale complex, is also focused on here. The sexual reproductive processes of heterothallic strains are controlled by two multifunctional sex pheromones, PR-IP and PR-IP Inducer, which independently promote multiple steps in conjugation at the appropriate times through different induction mechanisms. The molecules involved in sexual reproduction and sex determination have also been characterized.

Regulation by Light of Chemotaxis to Nitrite during the Sexual Life Cycle in Chlamydomonas reinhardtii

Nitrite plays an important role in the nitrogen metabolism of most cells, including Chlamydomonas reinhardtii. We have shown that vegetative cells of C. reinhardtii are attracted by nitrite. The Nia1nit2 mutant with defects in genes encoding the nitrate reductase and regulatory protein NIT2 respectively was found to exhibit normal chemotaxis to nitrite. The data suggest that chemotaxis events appear to be specific and independent of those involved in nitrate assimilation. Unlike vegetative cells and noncompetent pregametes, mature gametes did not show chemotaxis to nitrite. Just like gamete formation, the change in chemotaxis mode is controlled by the sequential action of two environmental cues, removal of nitrogen from the medium and light. Comparative analysis of wild-type and RNAi strains with reduced level of phototropin has indicated that switch-off of chemotaxis towards nitrite is dependent on phototropin. The studies revealed individual elements of the phototropin-dependent signal transduction pathway involved in the blue-light-controlled change in chemotaxis mode of C. reinhardtii during gamete formation: three protein kinases, one operating against signal flux and two that promote signal transduction. We have proposed a working model for the signaling pathway by which blue light controls chemotaxis towards attractants, which are nitrogen sources, during pregamete-to-gamete conversion of C. reinhardtii.

Regulation of cilium length and intraflagellar transport

Primary cilia are highly conserved sensory organelles that extend from the surface of almost all vertebrate cells. The importance of cilia is evident from their involvement in many diseases, called ciliopathies. Primary cilia contain a microtubular axoneme that is used as a railway for transport of both structural components and signaling proteins. This transport machinery is called intraflagellar transport (IFT). Cilia are dynamic organelles whose presence on the cell surface, morphology, length and function are highly regulated. It is clear that the IFT machinery plays an important role in this regulation. However, it is not clear how, for example environmental cues or cell fate decisions are relayed to modulate IFT and cilium morphology or function. This chapter presents an overview of molecules that have been shown to regulate cilium length and IFT. Several examples where signaling modulates IFT and cilium function are used to discuss the importance of these systems for the cell and for understanding of the etiology of ciliopathies.

Gsp1 triggers the sexual developmental program including inheritance of chloroplast DNA and mitochondrial DNA in Chlamydomonas reinhardtii

The isogamous green alga Chlamydomonas reinhardtii has emerged as a premier model for studying the genetic regulation of fertilization and sexual development. A key regulator is known to be a homeoprotein gene, GAMETE-SPECIFIC PLUS1 (GSP1), which triggers the zygotic program. In this study, we isolated a mutant, biparental31 (bp31), which lacks GSP1. bp31 mt+ gametes fuse normally to form zygotes, but the sexual development of the resulting diploid cell is arrested and pellicle/zygospore/tetrad formation is abolished. The uniparental inheritance of chloroplast (cp) and mitochondrial (mt) DNA (cytoplasmic inheritance) was also impaired. bp31 has a deletion of ∼60 kb on chromosome 2, including GSP1. The mutant phenotype was not rescued by transformation with GSP1 alone but could be rescued by the cotransformation with GSP1 and another gene, INOSITOL MONOPHOSPHATASE-LIKE1, which is involved in various cellular processes, including the phosphatidylinositol signaling pathway. This study confirms the importance of Gsp1 in mediating the zygotic program, including the uniparental inheritance of cp/mtDNA. Moreover, the results also suggest a role for inositol metabolism in the sexual developmental program.

Composition and sensory function of the trypanosome flagellar membrane

A cilium is an extension of the cell that contains an axonemal complex of microtubules and associated proteins bounded by a membrane which is contiguous with the cell body membrane. Cilia may be nonmotile or motile, the latter having additional specific roles in cell or fluid movement. The term flagellum refers to the motile cilium of free-living single cells (e.g. bacteria, archaea, spermatozoa, and protozoa). In eukaryotes, both nonmotile and motile cilia possess sensory functions. The ciliary interior (cilioplasm) is separated from the cytoplasm by a selective barrier that prevents passive diffusion of molecules between the two domains. The sensory functions of cilia reside largely in the membrane and signals generated in the cilium are transduced into a variety of cellular responses. In this review we discuss the structure and biogenesis of the cilium, with special attention to the trypanosome flagellar membrane, its lipid and protein composition and its proposed roles in sensing and signaling.

Chlamydomonas mutants display reversible deficiencies in flagellar beating and axonemal assembly

Axonemal complexes in flagella are largely prepackaged in the cell body. As such, one mutation often results in the absence of the co-assembled components and permanent motility deficiencies. For example, a Chlamydomonas mutant defective in RSP4 in the radial spoke (RS), which is critical for bend propagation, has paralyzed flagella that also lack the paralogue RSP6 and three additional RS proteins. Intriguingly, recent studies showed that several mutant strains contain a mixed population of swimmers and paralyzed cells despite their identical genetic background. Here we report a cause underlying these variations. Two new mutants lacking RSP6 swim processively and other components appear normally assembled in early log phase indicating that, unlike RSP4, this paralogue is dispensable. However, swimmers cannot maintain the typical helical trajectory and reactivated cell models tend to spin. Interestingly the motile fraction and the spokehead content dwindle during stationary phase. These results suggest that (1) intact RS is critical for maintaining the rhythm of oscillatory beating and thus the helical trajectory; (2) assembly of the axonemal complex with subtle defects is less efficient and the inefficiency is accentuated in compromised conditions, leading to reversible dyskinesia. Consistently, several organisms only possess one RSP4/6 gene. Gene duplication in Chlamydomonas enhances RS assembly to maintain optimal motility in various environments.

The Trypanosoma brucei flagellum: moving parasites in new directions

African trypanosomes are devastating human and animal pathogens. Trypanosoma brucei rhodesiense and T. b. gambiense subspecies cause the fatal human disease known as African sleeping sickness. It is estimated that several hundred thousand new infections occur annually and the disease is fatal if untreated. T. brucei is transmitted by the tsetse fly and alternates between bloodstream-form and insect-form life cycle stages that are adapted to survive in the mammalian host and the insect vector, respectively. The importance of the flagellum for parasite motility and attachment to the tsetse fly salivary gland epithelium has been appreciated for many years. Recent studies have revealed both conserved and novel features of T. brucei flagellum structure and composition, as well as surprising new functions that are outlined here. These discoveries are important from the standpoint of understanding trypanosome biology and identifying novel drug targets, as well as for advancing our understanding of fundamental aspects of eukaryotic flagellum structure and function.

The conserved plant sterility gene HAP2 functions after attachment of fusogenic membranes in Chlamydomonas and Plasmodium gametes

The cellular and molecular mechanisms that underlie species-specific membrane fusion between male and female gametes remain largely unknown. Here, by use of gene discovery methods in the green alga Chlamydomonas, gene disruption in the rodent malaria parasite Plasmodium berghei, and distinctive features of fertilization in both organisms, we report discovery of a mechanism that accounts for a conserved protein required for gamete fusion. A screen for fusion mutants in Chlamydomonas identified a homolog of HAP2, an Arabidopsis sterility gene. Moreover, HAP2 disruption in Plasmodium blocked fertilization and thereby mosquito transmission of malaria. HAP2 localizes at the fusion site of Chlamydomonas minus gametes, yet Chlamydomonas minus and Plasmodium hap2 male gametes retain the ability, using other, species-limited proteins, to form tight prefusion membrane attachments with their respective gamete partners. Membrane dye experiments show that HAP2 is essential for membrane merger. Thus, in two distantly related eukaryotes, species-limited proteins govern access to a conserved protein essential for membrane fusion.

Gametic cell adhesion and fusion in the unicellular alga Chlamydomonas

Differentiation of vegetative cells of the haploid eukaryote Chlamydomonas is dependent on environmental conditions. Upon depletion of nitrogen and exposure to light, vegetative cells undergo a mitotic division, generating gametes that are either mating-type plus (mt[+]) or mating-type minus (mt[-]). As gametes of opposite mating type encounter one another, an initial adhesive interaction mediated by flagella induces a signal transduction pathway that results in activation of gametes. Gametic activation results in the exposure of previously cryptic regions of the plasma membrane (mating structures) that contain the molecules required for gametic cell adhesion and fusion. Recent studies have identified new steps in this signal transduction pathway, including the tyrosine phosphorylation of a cyclic guanosine monophosphate-dependent protein kinase, a requirement for a novel microtubular motility known as intraflagellar transport, and a mt(+)-specific molecule that mediates adhesion between mating structures.

Changes in photoreceptor currents and their sensitivity to the chemoeffector tryptone during gamete mating in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii Dangeard generates photoreceptor currents (PCs) upon light excitation. These currents play a key role in the signal transduction chain for photomotility responses. We have previously found that inhibition of PCs by tryptone occurs only in gametes that display chemotaxis toward this agent, and is not observed in chemotactically insensitive vegetative cells. Here we show that the sensitivity to tryptone is characteristic of gametes of both mating types, and examine the influence of gamete mating on PCs and their sensitivity to tryptone. The amplitude of PCs increases after cell fusion, but the sensitivity of these currents to tryptone decreases upon flagellar adhesion and/or an increase in the intracellular cAMP concentration. Net chemotaxis toward tryptone is reduced in young zygotes compared to gametes. We conclude that gamete mating leads to rapid inactivation of a gamete-specific chemosensory system.

Intraflagellar Transport Particles Participate Directly in Cilium-Generated Signaling in Chlamydomonas

Primary cilia are widely used for signal transduction during development and in homeostasis and are assembled and maintained by intraflagellar transport (IFT). Here, we have dissected the role of IFT in signaling within the flagella (structural and functional counterparts of cilia) of the biflagellated green alga Chlamydomonas. Using a conditional IFT mutant enables us to deplete the IFT machinery from intact, existing flagella. We identify a cGMP-dependent protein kinase (CrPKG) within flagella as the substrate of a protein tyrosine kinase activated by flagellar adhesion during fertilization. We demonstrate that flagellar adhesion stimulates association of CrPKG with a new flagellar compartment. Moreover, formation of the compartment requires IFT, and IFT particles themselves are part of the compartment. Our results lead to a model in which the IFT machinery is required not only for assembling cilia and flagella but also for organizing a signaling pathway within the organelles during cilium-generated signaling.

Flagellar adhesion between mating type plus and mating type minus gametes activates a flagellar protein-tyrosine kinase during fertilization in Chlamydomonas

When Chlamydomonas gametes of opposite mating type are mixed together, flagellar adhesion through sex-specific adhesion molecules triggers a transient elevation of intracellular cAMP, leading to gamete activation in preparation for cell-cell fusion and zygote formation. Here, we have identified a protein-tyrosine kinase (PTK) activity that is stimulated by flagellar adhesion. We determined that the protein-tyrosine kinase inhibitor genistein inhibited fertilization, and that fertilization was rescued by dibutyryl cAMP, indicating that the genistein-sensitive step was upstream of the increase in cAMP. Incubation with ATP of flagella isolated from non-adhering and adhering gametes followed by SDS-PAGE and immunoblotting with anti-phosphotyrosine antibodies showed that adhesion activated a flagellar PTK that phosphorylated a 105-kDa flagellar protein. Assays using an exogenous protein-tyrosine kinase substrate confirmed that the activated PTK could be detected only in flagella isolated from adhering gametes. Our results indicate that stimulation of the PTK is a very early event during fertilization. Activation of the PTK was blocked when gametes underwent flagellar adhesion in the presence of the protein kinase inhibitor staurosporine, but not in the presence of the cyclic nucleotide-dependent protein kinase inhibitor, H8, which (unlike staurosporine) does not block the increases in cAMP. In addition, incubation of gametes of a single mating type in dibutyryl cAMP failed to activate the PTK. Finally, flagella adhesion between plus and minus fla10-1 gametes, which have a temperature-sensitive lesion in the microtubule motor protein kinesin-II, failed to activate the PTK at elevated temperatures. Our results show that kinesin-II is essential for coupling flagellar adhesion to activation of a flagellar PTK and cAMP generation during fertilization in Chlamydomonas.

Protein transport and signal transduction during fertilization in chlamydomonas

Fertilization in Chlamydomonas begins with flagellar adhesion between mating type plus and mating type minus gametes and is consummated within minutes by zygote formation. Once fusion occurs, the newly merged gametes cease existence as distinct entities, and the diploid zygote immediately initiates transcription of zygote-specific genes. Accomplishing fertilization within such a short time requires the rapid and signaled movement of pre-existing membrane and cytoplasmic proteins between and within several cellular compartments. Generation within the adhering flagella of the initial signals for protein movement, as well as movement itself of at least one cytoplasmic protein from the cell body to the flagella, depend on the microtubule motor, kinesin-II and presumably on intraflagellar transport (IFT). Adhesion and fusion of the two gametes depend on a second translocation event, the movement of an adhesion/fusion protein onto the surface of a rapidly elongating, microvillous-like fusion organelle. Finally, the merging of the two separate gametes, each containing sex-specific proteins, into a single cell allows the formerly separate proteins to form new interactions that regulate zygote development. Two proteins - a nuclease and a homeodomain protein - which were present only in the plus gamete, are 'delivered' to the cytoplasm of the zygote during gamete fusion. The nuclease is selectively imported into the minus chloroplast, where it degrades the chloroplast DNA, thereby ensuring uniparental inheritance of plus chloroplast traits. The homeodomain protein binds with an as yet unidentified protein delivered by the minus gamete, and the new complex activates transcription of zygote-specific genes.

The Chlamydomonas Fus1 protein is present on the mating type plus fusion organelle and required for a critical membrane adhesion event during fusion with minus gametes

The molecular mechanisms of the defining event in fertilization, gamete fusion, remain poorly understood. The FUS1 gene in the unicellular, biflagellated green alga Chlamydomonas is one of the few sex-specific eukaryotic genes shown by genetic analysis to be essential for gamete fusion during fertilization. In Chlamydomonas, adhesion and fusion of the plasma membranes of activated mt+ and mt- gametes is accomplished via specialized fusion organelles called mating structures. Herein, we identify the endogenous Fus1 protein, test the idea that Fus1 is at the site of fusion, and identify the step in fusion that requires Fus1. Our results show that Fus1 is a approximately 95-kDa protein present on the external surface of both unactivated and activated mt+ gametes. Bioassays indicate that adhesion between mating type plus and mating type minus fusion organelles requires Fus1 and that Fus1 is functional only after gamete activation. Finally, immunofluorescence demonstrates that the Fus1 protein is present as an apical patch on unactivated gametes and redistributes during gamete activation over the entire surface of the microvillous-like activated plus mating structure, the fertilization tubule. Thus, Fus1 is present on mt+ gametes at the site of cell-cell fusion and essential for an early step in the fusion process.

Kinesin-II is required for flagellar sensory transduction during fertilization in Chlamydomonas

The assembly and maintenance of eucaryotic flagella and cilia depend on the microtubule motor, kinesin-II. This plus end-directed motor carries intraflagellar transport particles from the base to the tip of the organelle, where structural components of the axoneme are assembled. Here we test the idea that kinesin-II also is essential for signal transduction. When mating-type plus (mt+) and mating-type minus (mt-) gametes of the unicellular green alga Chlamydomonas are mixed together, binding interactions between mt+ and mt- flagellar adhesion molecules, the agglutinins, initiate a signaling pathway that leads to increases in intracellular cAMP, gamete activation, and zygote formation. A critical question in Chlamydomonas fertilization has been how agglutinin interactions are coupled to increases in intracellular cAMP. Recently, fla10 gametes with a temperature-sensitive defect in FLA10 kinesin-II were found to not form zygotes at the restrictive temperature (32 degrees C). We found that, although the rates and extents of flagellar adhesion in fla10 gametes at 32 degrees C are indistinguishable from wild-type gametes, the cells do not undergo gamete activation. On the other hand, fla10 gametes at 32 degrees C regulated agglutinin location and underwent gamete fusion when the cells were incubated in dibutyryl cAMP, indicating that their capacity to respond to the cAMP signal was intact. We show that the cellular defect in the fla10 gametes at 32 degrees C is a failure to undergo increases in cAMP during flagella adhesion. Thus, in addition to being essential for assembly and maintenance of the structural components of flagella, kinesin-II/intraflagellar transport plays a role in sensory transduction in these organelles.

Signal transduction during fertilization in the unicellular green alga, Chlamydomonas

Sexual reproduction in the green alga, Chlamydomonas, is regulated by environmental conditions and by cell-cell interactions. After gametogenesis, flagellar adhesion between gametes triggers gamete activation, leading to cell fusion and zygote formation. Recent studies have identified new molecular events that underlie signal transduction during Chlamydomonas fertilization, including expression of a sex-determining protein, phosphorylation of a homeodomain protein, activity of a kinesin II and regulated translocation of an aurora/Ip11-like protein kinase from the cell body to the flagella.

Regulated targeting of a protein kinase into an intact flagellum. An aurora/Ipl1p-like protein kinase translocates from the cell body into the flagella during gamete activation in chlamydomonas

In the green alga Chlamydomonas reinhardtii flagellar adhesion between gametes of opposite mating types leads to rapid cellular changes, events collectively termed gamete activation, that prepare the gametes for cell-cell fusion. As is true for gametes of most organisms, the cellular and molecular mechanisms that underlie gamete activation are poorly understood. Here we report on the regulated movement of a newly identified protein kinase, Chlamydomonas aurora/Ipl1p-like protein kinase (CALK), from the cell body to the flagella during gamete activation. CALK encodes a protein of 769 amino acids and is the newest member of the aurora/Ipl1p protein kinase family. Immunoblotting with an anti-CALK antibody showed that CALK was present as a 78/80-kDa doublet in vegetative cells and unactivated gametes of both mating types and was localized primarily in cell bodies. In cells undergoing fertilization, the 78-kDa CALK was rapidly targeted to the flagella, and within 5 min after mixing gametes of opposite mating types, the level of CALK in the flagella began to approach levels normally found in the cell body. Protein synthesis was not required for targeting, indicating that the translocated CALK and the cellular molecules required for its movement are present in unactivated gametes. CALK was also translocated to the flagella during flagellar adhesion of nonfusing mutant gametes, demonstrating that cell fusion was not required for movement. Finally, the requirement for flagellar adhesion could be bypassed; incubation of cells of a single mating type in dibutyryl cAMP led to CALK translocation to flagella in gametes but not vegetative cells. These experiments document a new event in gamete activation in Chlamydomonas and reveal the existence of a mechanism for regulated translocation of molecules into an intact flagellum.

Flagellar adhesion between mt(+) and mt(-) Chlamydomonas gametes regulates phosphorylation of the mt(+)-specific homeodomain protein GSP1

During fertilization in Chlamydomonas, flagellar adhesion between mt(+) and mt(-) gametes induces a cAMP-dependent signal transduction pathway that prepares the gametes for cell fusion and zygote formation. Previously, our laboratory identified a homeodomain protein (GSP1) whose expression was restricted to the cell bodies of mt(+) gametes and whose transcript level was up-regulated during flagellar adhesion. In this report, we describe a new form of GSP1 that appears early during gamete interactions. Immunoblot analysis showed that in addition to the 120-kDa form of GSP1 normally present in mt(+) gametes, a 122-kDa form was detected when the cells were mixed with mt(-) gametes. The more slowly migrating form of GSP1 was detectable within minutes after gametes were mixed together, and its appearance did not require new protein synthesis. Thus, the 122-kDa form represents a post-translational modification of the pre-existing 120-kDa form of GSP1. Moreover, conversion to the 122-kDa form did not require cell fusion. Although the 120-kDa form was expressed 10 h after vegetative cells were transferred to gametic induction medium, the 122-kDa form was detected only after mt(+) gametes were induced to undergo the sexual signaling that accompanies fertilization. Incubation of mt(+) gametes with dibutyryl cAMP led to the appearance of the 122-kDa form of GSP1, and the cyclic nucleotide-dependent protein kinase inhibitor H-8 inhibited the adhesion-induced conversion. Incubation of GSP1 immunoprecipitated from signaling mt(+) gametes with alkaline phosphatase showed that the conversion was due to phosphorylation. The results indicate that flagellar adhesion induces a rapid, cAMP-dependent phosphorylation of the homeodomain protein GSP1 early during fertilization in Chlamydomonas.

Cell adhesion-dependent inactivation of a soluble protein kinase during fertilization in Chlamydomonas

Within seconds after the flagella of mt+ and mt- Chlamydomonas gametes adhere during fertilization, their flagellar adenylyl cyclase is activated several fold and preparation for cell fusion is initiated. Our previous studies indicated that early events in this pathway, including control of adenylyl cyclase, are regulated by phosphorylation and dephosphorylation. Here, we describe a soluble, flagellar protein kinase activity that is regulated by flagellar adhesion. A 48-kDa, soluble flagellar protein was consistently phosphorylated in an in vitro assay in flagella isolated from nonadhering mt+ and mt- gametes, but not in flagella isolated from mt+ and mt- gametes that had been adhering for 1 min. Although the 48-kDa protein was present in the flagella isolated from adhering gametes, we demonstrate that its protein kinase was inactivated by flagellar adhesion. Immunoblot analysis and inhibitor studies indicate that the 48-kDa protein in nonadhering gametes is phosphorylated by a protein tyrosine kinase. In vivo experiments showing that the protein tyrosine phosphatase inhibitor sodium orthovanadate inhibits fertilization suggest that protein dephosphorylation may be required for signal transduction. The 48-kDa protein and its protein kinase may be among the first elements of a novel signalling pathway that couples interaction of flagellar adhesion molecules to gamete activation.

Molecular cloning of a protein kinase whose phosphorylation is regulated by genetic adhesion during Chlamydomonas fertilization

Fertilization in Chlamydomonas is initiated by adhesive interactions between gametes of opposite mating types through flagellar glycoproteins called agglutinins. Interactions between these cell adhesion molecules signal for the activation of adenylyl cyclase through an interplay of protein kinases and ultimately result in formation of a diploid zygote. One of the early events during adhesion-induced signal transduction is the rapid inactivation of a flagellar protein kinase that phosphorylates a 48-kDa protein in the flagella. We report the biochemical and molecular characterization of the 48-kDa protein. Experiments using a bacterially expressed fusion protein show that the 48-kDa protein is capable of autophosphorylation on serine and tyrosine and phosphorylation of bovine beta-casein on serine, confirming that the 48-kDa protein itself has protein kinase activity. This protein kinase exhibits limited homology to members of the eukaryotic protein kinase superfamily and may be an important element in a signaling pathway in fertilization.

Increased transcript levels of a methionine synthase during adhesion-induced activation of Chlamydomonas reinhardtii gametes

Chlamydomonas gametes of opposite mating types interact through flagellar adhesion molecules called agglutinins leading to a signal transduction cascade that induces cell wall loss and activation of mating structures along with other cellular responses that ultimately result in zygote formation. To identify molecules involved in these complex cellular events, we have employed subtractive and differential hybridization with cDNA from mt+ gametes activated for fertilization and non-signaling, vegetative (non-gametic) cells. We identified 55 cDNA clones whose transcripts were regulated in activated gametes. Here we report the molecular cloning and characterization of the complementary DNA (cDNA) for one clone whose transcripts in activated gametes were several-fold higher than in normal gametes. Regulation of the transcript was not related simply to protein synthesis because it was not increased in cells synthesizing new cell wall proteins. The cDNA contained a single open reading frame (ORF) of 815 amino acids encoding a polypeptide of calculated relative mass of 87 kDa. Database search analysis and sequence alignment indicated that the deduced amino acid sequence exhibited 42% identity and 62% similarity to a class of prokaryotic methyl transferases (5-methyltetrahydrofolate-homocysteine methyl transferase; EC 2.1.1.14) known to be involved in the terminal step of de novo biosynthesis of methionine. This enzyme catalyzes transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine resulting in methionine formation. Affinity-purified polyclonal antibodies raised against a bacterially produced GST-fusion protein identified a 85 kDa soluble protein in Chlamydomonas gametes. Southern blot hybridization indicated that the enzyme is encoded by a single-copy gene. The evidence presented in this paper raises the possibility that, in addition to its participation in de novo biosynthesis and regeneration of methionine, Chlamydomonas methionine synthase may play a role in adhesion-induced events during fertilization.

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.

Dissection of eukaryotic transmembrane signalling using Chlamydomonas

Novel insights and surprises are often generated when investigators choose an organism that permits a new approach to a problem. For example, secretory and cell-cycle mutants in yeast have provided quantum leaps in elucidating these processes. Similarly, genetic systems are providing exciting new insights into signal transduction. The 'green yeast' Chlamydomonas has the potential to be a particularly rich organism for genetic analysis of signal transduction because, although unicellular, it has several interesting behaviours, which are discussed in this article by Lynne Quarmby and Criss Hartzell. Phototaxis results from the transduction of a light signal received by the eyespot to changes in flagellar beat. The mating reactions, which culminate in the fusion of gametes, are initiated in response to adhesion of flagellar proteins. Deflagellation, or flagellar shedding, is an acute response to a variety of stimuli. Molecular genetic analysis of behavioural mutants is providing new directions for understanding signal integration and segregation.