Activation of adenylyl cyclase in Chlamydomonas reinhardtii by adhesion and by heat

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.

HAP2-Mediated Gamete Fusion: Lessons From the World of Unicellular Eukaryotes

Most, if not all the cellular requirements for fertilization and sexual reproduction arose early in evolution and are retained in extant lineages of single-celled organisms including a number of important model organism species. In recent years, work in two such species, the green alga, Chlamydomonas reinhardtii, and the free-living ciliate, Tetrahymena thermophila, have lent important new insights into the role of HAP2/GCS1 as a catalyst for gamete fusion in organisms ranging from protists to flowering plants and insects. Here we summarize the current state of knowledge around how mating types from these algal and ciliate systems recognize, adhere and fuse to one another, current gaps in our understanding of HAP2-mediated gamete fusion, and opportunities for applying what we know in practical terms, especially for the control of protozoan parasites.

Adenylyl cyclase 5 deficiency reduces renal cyclic AMP and cyst growth in an orthologous mouse model of polycystic kidney disease

Cyclic AMP promotes cyst growth in polycystic kidney disease (PKD) by stimulating cell proliferation and fluid secretion. Previously, we showed that the primary cilium of renal epithelial cells contains a cAMP regulatory complex comprising adenylyl cyclases 5 and 6 (AC5/6), polycystin-2, A-kinase anchoring protein 150, protein kinase A, and phosphodiesterase 4C. In Kif3a mutant cells that lack primary cilia, the formation of this regulatory complex is disrupted and cAMP levels are increased. Inhibition of AC5 reduces cAMP levels in Kif3a mutant cells, suggesting that AC5 may mediate the increase in cAMP in PKD. Here, we examined the role of AC5 in an orthologous mouse model of PKD caused by kidney-specific ablation of Pkd2. Knockdown of AC5 with siRNA attenuated the increase in cAMP levels in Pkd2-deficient renal epithelial cells. Levels of cAMP and AC5 mRNA transcripts were elevated in the kidneys of mice with collecting duct-specific ablation of Pkd2. Compared with Pkd2 single mutant mice, AC5/Pkd2 double mutant mice had less kidney enlargement, lower cyst index, reduced kidney injury, and improved kidney function. Importantly, cAMP levels and cAMP-dependent signaling were reduced in the kidneys of AC5/Pkd2 double mutant compared to the kidneys of Pkd2 single mutant mice. Additionally, we localized endogenous AC5 in the primary cilium of renal epithelial cells and showed that ablation of AC5 reduced ciliary elongation in the kidneys of Pkd2 mutant mice. Thus, AC5 contributes importantly to increased renal cAMP levels and cyst growth in Pkd2 mutant mice, and inhibition of AC5 may be beneficial in the treatment of PKD.

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.

Whole genome sequencing identifies a deletion in protein phosphatase 2A that affects its stability and localization in Chlamydomonas reinhardtii

Whole genome sequencing is a powerful tool in the discovery of single nucleotide polymorphisms (SNPs) and small insertions/deletions (indels) among mutant strains, which simplifies forward genetics approaches. However, identification of the causative mutation among a large number of non-causative SNPs in a mutant strain remains a big challenge. In the unicellular biflagellate green alga Chlamydomonas reinhardtii, we generated a SNP/indel library that contains over 2 million polymorphisms from four wild-type strains, one highly polymorphic strain that is frequently used in meiotic mapping, ten mutant strains that have flagellar assembly or motility defects, and one mutant strain, imp3, which has a mating defect. A comparison of polymorphisms in the imp3 strain and the other 15 strains allowed us to identify a deletion of the last three amino acids, Y313F314L315, in a protein phosphatase 2A catalytic subunit (PP2A3) in the imp3 strain. Introduction of a wild-type HA-tagged PP2A3 rescues the mutant phenotype, but mutant HA-PP2A3 at Y313 or L315 fail to rescue. Our immunoprecipitation results indicate that the Y313, L315, or YFLΔ mutations do not affect the binding of PP2A3 to the scaffold subunit, PP2A-2r. In contrast, the Y313, L315, or YFLΔ mutations affect both the stability and the localization of PP2A3. The PP2A3 protein is less abundant in these mutants and fails to accumulate in the basal body area as observed in transformants with either wild-type HA-PP2A3 or a HA-PP2A3 with a V310T change. The accumulation of HA-PP2A3 in the basal body region disappears in mated dikaryons, which suggests that the localization of PP2A3 may be essential to the mating process. Overall, our results demonstrate that the terminal YFL tail of PP2A3 is important in the regulation on Chlamydomonas mating.

A photochromic histidine kinase rhodopsin (HKR1) that is bimodally switched by ultraviolet and blue light

Rhodopsins are light-activated chromoproteins that mediate signaling processes via transducer proteins or promote active or passive ion transport as ion pumps or directly light-activated channels. Here, we provide spectroscopic characterization of a rhodopsin from the Chlamydomonas eyespot. It belongs to a recently discovered but so far uncharacterized family of histidine kinase rhodopsins (HKRs). These are modular proteins consisting of rhodopsin, a histidine kinase, a response regulator, and in some cases an effector domain such as an adenylyl or guanylyl cyclase, all encoded in a single protein as a two-component system. The recombinant rhodopsin fragment, Rh, of HKR1 is a UVA receptor (λ(max) = 380 nm) that is photoconverted by UV light into a stable blue light-absorbing meta state Rh-Bl (λ(max) = 490 nm). Rh-Bl is converted back to Rh-UV by blue light. Raman spectroscopy revealed that the Rh-UV chromophore is in an unusual 13-cis,15-anti configuration, which explains why the chromophore is deprotonated. The excited state lifetime of Rh-UV is exceptionally stable, probably caused by a relatively unpolar retinal binding pocket, converting into the photoproduct within about 100 ps, whereas the blue form reacts 100 times faster. We propose that the photochromic HKR1 plays a role in the adaptation of behavioral responses in the presence of UVA light.

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.

Uniparental inheritance of cpDNA and the genetic control of sexual differentiation in Chlamydomonas reinhardtii

An intriguing feature of most eukaryotes is that chloroplast (cp) and mitochondrial (mt) genomes are inherited almost exclusively from one parent. Uniparental inheritance of cp/mt genomes was long thought to be a passive outcome, based on the fact that eggs contain multiple numbers of organelles, while male gametes contribute,at best, only a few cp/mtDNA. However, the process is likely to be more dynamic because uniparental inheritance occurs in organisms that produce gametes of identical sizes (isogamous). In Chlamydomonas reinhardtii,the uniparental inheritance of cp/mt genomes is achieved by a series of mating type-controlled events that actively eliminate the mating type minus (mt-) cpDNA.The method by which Chlamydomonas selectively degrades mt- cpDNA has long fascinated researchers, and is the subject of this review.

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.

Sex determination in Chlamydomonas

The sex-determination system of the unicellular green alga, Chlamydomonas reinhardtii, is governed by genes in the mating-type (MT) locus and entails additional genes located in autosomes. Gene expression is initiated by nitrogen starvation, and cells differentiate into plus or minus gametes within 6h. Reviewed is our current understanding of gametic differentiation and fertilization, initiation of zygote development, and the uniparental inheritance of organelle genomes.

Gametogenesis in the Chlamydomonas reinhardtii minus mating type is controlled by two genes, MID and MTD1

In the unicellular algae Chlamydomonas reinhardtii, the plus and minus mating types are controlled by a complex locus, MT, where the dominant MID gene in the MT(-) locus has been shown to be necessary for expression of minus-specific gamete-specific genes in response to nitrogen depletion. We report studies on MID expression patterns during gametogenesis and on a second gene unique to the MT(-) locus, MTD1. Vegetative cells express basal levels of MID. An early activation of MID transcription after nitrogen removal, and its sequence similarity to plant RWP-RK proteins involved in nitrogen-responsive processes, suggest that Mid conformation/activity may be nitrogen sensitive. A second stage of MID upregulation correlates with the acquisition of mating ability in minus gametes. Knockdown of MTD1 by RNAi in minus strains results in a failure to differentiate into gametes of either mating type after nitrogen deprivation. We propose that intermediate Mid levels are sufficient to activate MTD1 transcription and to repress plus gamete-specific genes and that MTD1 expression in turn allows the threshold-level MID expression needed to turn on minus gamete-specific genes. We further propose that an MTD1-equivalent system, utilizing at least one gene product encoded in the MT(+) locus, is operant during plus gametogenesis.

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.

The flagellar motility of Chlamydomonas pf25 mutant lacking an AKAP-binding protein is overtly sensitive to medium conditions

Radial spokes are a conserved axonemal structural complex postulated to regulate the motility of 9 + 2 cilia and flagella via a network of phosphoenzymes and regulatory proteins. Consistently, a Chlamydomonas radial spoke protein, RSP3, has been identified by RII overlays as an A-kinase anchoring protein (AKAP) that localizes the cAMP-dependent protein kinase (PKA) holoenzyme by binding to the RIIa domain of PKA RII subunit. However, the highly conserved docking domain of PKA is also found in the N termini of several AKAP-binding proteins unrelated to PKA as well as a 24-kDa novel spoke protein, RSP11. Here, we report that RSP11 binds to RSP3 directly in vitro and colocalizes with RSP3 toward the spoke base near outer doublets and dynein motors in axonemes. Importantly, RSP11 mutant pf25 displays a spectrum of motility, from paralysis with flaccid or twitching flagella as other spoke mutants to wildtype-like swimming. The wide range of motility changes reversibly depending on the condition of liquid media without replacing defective proteins. We postulate that radial spokes use the RIIa/AKAP module to regulate ciliary and flagellar beating; absence of the spoke RIIa protein exposes a medium-sensitive regulatory mechanism that is not obvious in wild-type Chlamydomonas.

Mating-induced shedding of cell walls, removal of walls from vegetative cells, and osmotic stress induce presumed cell wall genes in Chlamydomonas

The first step in sexual differentiation of the unicellular green alga Chlamydomonas reinhardtii is the formation of gametes. Three genes, GAS28, GAS30, and GAS31, encoding Hyp-rich glycoproteins that presumably are cell wall constituents, are expressed in the late phase of gametogenesis. These genes, in addition, are activated by zygote formation and cell wall removal and by the application of osmotic stress. The induction by zygote formation could be traced to cell wall shedding prior to gamete fusion since it was seen in mutants defective in cell fusion. However, it was absent in mutants defective in the initial steps of mating, i.e. in flagellar agglutination and in accumulation of adenosine 3',5'-cyclic monophosphate in response to this agglutination. Induction of the three GAS genes was also observed when cultures were exposed to hypoosmotic or hyperosmotic stress. To address the question whether the induction seen upon cell wall removal from both gametes and vegetative cells was elicited by osmotic stress, cell wall removal was performed under isosmotic conditions. Also under such conditions an activation of the genes was observed, suggesting that the signaling pathway(s) is (are) activated by wall removal itself.

Cyclic AMP levels in several macroalgae and their relation to light quantity and quality

Total cAMP levels were measured in the macroalgae Dictyota dichotoma, Gelidium sesquipedale and Ulva rigida under different light conditions in order to study its regulation either by phytochrome or photosynthesis. Incubation in red or far-red light did not promote a phytochrome-like response; instead, it showed a synergistic effect upon cAMP accumulation. cAMP levels seemed to depend on the amount of energy applied. The correlation between photosynthetic oxygen evolution and cAMP variations at sub-saturating white light irradiance pointed to photosynthetic electron transport as involved in the regulation of cAMP accumulation at least in G. sesquipedale and U. rigida. Inhibitors of thylakoidal and mitochondrial electron transport chains reduced cAMP levels in 70 to 99%. We conclude that cAMP accumulation could be regulated by photosynthetic activity rather than phytochrome in the macroalgae studied.

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.

Light regulation of cyclic-AMP levels in the red macroalga Porphyra leucosticta

Total cyclic-3'-5'-adenosine monophosphate (cAMP) levels were measured in the gametophyte of the red macroalga Porphyra leucosticta under different light conditions in order to study its regulation by phytochrome or photosynthesis. cAMP levels were relatively low when samples were incubated in darkness, or exposed to red or far-red light. Irradiation with red+far-red light induced a moderate increase in cAMP levels, while white light induced a pronounced increase in cAMP levels. When incubated under increasing white light irradiance, cAMP levels closely followed the increase in photosynthetic oxygen evolution rate, suggesting a direct relationship between photosynthesis and cAMP accumulation. cAMP levels were not dependent on cellular ATP concentration, as inhibitors of ATP synthesis did not significantly affect cAMP levels in light. We conclude that cAMP depends on photosynthetic activity regardless of ATP synthesis and concentration or phytochrome activity.

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.

Regulation of flagellar length in Chlamydomonas

The length of eukaryotic cilia and flagella depends on the cell cycle-regulated assembly and disassembly of at least 9 doublet and 2 central microtubules, their associated proteins, and the surrounding membrane. In light-synchronized Chlamydomonas cells, flagella assembled to 10-14 microm in length near the beginning of the light period and they disassembled prior to cell division, during the dark period. Flagella on light-synchronized pf18 Chlamydomonas mutants grew to 10-12 microm near the beginning of the light period but shortened by 50% or more by the end of the light period. Flagellar length was cell-cycle regulated: when flagella were amputated at various times during the light period, new flagella regenerated to the lengths of control cells at that time of the light cycle. The later in the cycle pf18 cells were deflagellated, the shorter were the regenerated flagella. Flagellar shortening was not affected, in either pf18 or wild-type (wt) cells, by inhibitors of protein synthesis or of microtubule assembly, so flagellar length cannot depend on protein turnover. Shortening in pf18 was attenuated by Li+, which stimulated flagellar growth in wt cells, by red light, by protein kinase inhibitors, and by the Ca2+ channel blockers La3+ and Cd2+. Shortening was increased by cAMP, Na+, K+, and EGTA. Ca2+-CAM blockers did not affect pf18 shortening but they increased shortening in wt and fa1 cells. We propose that flagellar length is regulated by a signal transduction pathway that is sensitive to Ca2+ levels and red light.

The Chlamydomonas zygospore: mutant strains of Chlamydomonas monoica blocked in zygospore morphogenesis comprise 46 complementation groups

Chlamydomonas monoica undergoes homothallic sexual reproduction in response to nitrogen starvation. Mating pairs are established in clonal culture via flagellar agglutination and fuse by way of activated mating structures to form the quadriflagellate zygote. The zygote further matures into a dormant diploid zygospore through a series of events that we collectively refer to as zygosporulation. Mutants that arrest development prior to the completion of zygosporulation have been obtained through the use of a variety of mutagens, including ultraviolet irradiation, 5-fluorodeoxyuridine, ethyl methanesulfonate, and methyl methanesulfonate. Complementation analysis indicates that the present mutant collection includes alleles affecting 46 distinct zygote-specific functions. The frequency with which alleles at previously defined loci have been recovered in the most recent mutant searches suggests that as many as 30 additional zygote-specific loci may still remain to be identified. Nevertheless, the present collection should provide a powerful base for ultrastructural, biochemical, and molecular analysis of zygospore morphogenesis and dormancy 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.

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

Interactions between adhesion molecules, agglutinins, on the surfaces of the flagella of mt+ and mt- gametes in Chlamydomonas rapidly generate a sexual signal, mediated by cAMP, that prepares the cells for fusion to form a zygote. The mechanism that couples agglutinin interactions to increased cellular levels of cAMP is unknown. In previous studies on the adenylyl cyclase in flagella of a single mating type (i.e., non-adhering flagella) we presented evidence that the gametic form of the enzyme, but not the vegetative form, was regulated by phosphorylation and dephosphorylation (Zhang, Y., E. M. Ross, and W. J. Snell. 1991. J. Biol. Chem. 266:22954-22959; Zhang, Y., and W. J. Snell. 1993. J. Biol. Chem. 268:1786-1791). In the present report we describe studies on regulation of flagellar adenylyl cyclase during adhesion in a cell-free system. The results show that the activity of gametic flagellar adenylyl cyclase is regulated by adhesion in vitro between flagella isolated from mt+ and mt- gametes. After mixing mt+ and mt- flagella together for 15 s in vitro, adenylyl cyclase activity was increased two- to threefold compared to that of the non-mixed (non-adhering), control flagella. This indicates that the regulation of gametic flagellar adenylyl cyclase during the early steps of fertilization is not mediated by signals from the cell body, but is a direct and primary response to interactions between mt+ and mt- agglutinins. By use of this in vitro assay, we discovered that 50 nM staurosporine (a protein kinase inhibitor) blocked adhesion-induced activation of adenylyl cyclase in vitro, while it had no effect on adenylyl cyclase activity of non-adhering gametic flagella. This same low concentration of staurosporine also inhibited adhesion-induced increases in vivo in cellular cAMP and blocked subsequent cellular responses to adhesion. Taken together, our results indicate that flagellar adenylyl cyclase in Chlamydomonas gametes is coupled to interactions between mt+ and mt- agglutinins by a staurosporine-sensitive activity, probably a protein kinase.

Two distinct, calcium-mediated, signal transduction pathways can trigger deflagellation in Chlamydomonas reinhardtii

The molecular machinery of deflagellation can be activated in detergent permeabilized Chlamydomonas reinhardtii by the addition of Ca2+ (Sanders, M. A., and J. L. Salisbury, 1989. J. Cell Biol. 108:1751-1760). This suggests that stimuli which induce deflagellation in living cells cause an increase in the intracellular concentration of Ca2+, but this has never been demonstrated. In this paper we report that the wasp venom peptide, mastoparan, and the permeant organic acid, benzoate, activate two different signalling pathways to trigger deflagellation. We have characterized each pathway with respect to: (a) the requirement for extracellular Ca2+; (b) sensitivity to Ca2+ channel blockers; and (c) 45Ca influx. We also report that a new mutant strain of C. reinhardtii, adf-1, is specifically defective in the acid-activated signalling pathway. Both signalling pathways appear normal in another mutant, fa-1, that is defective in the machinery of deflagellation (Lewin, R. and C. Burrascano. 1983. Experientia. 39:1397-1398; Sanders, M. A., and J. L. Salisbury. 1989. J. Cell Biol. 108:1751-1760). We conclude that mastoparan induces the release of an intracellular pool of Ca2+ whereas acid induces an influx of extracellular Ca2+ to activate the machinery of deflagellation.