Voltage-gated Ca2+ entry into Paramecium linked to intraciliary increase in cyclic GMP

Ion Channels of Cilia: Paramecium as a Model

Holotrichous ciliates, like Paramecium, swim through their aqueous environment by beating their many cilia. They can alter swimming speed and direction, which seems to have mesmerized early microscopists of the 1600's. We know from extensive and elegant physiological studies and generation of mutants that these cells can be considered little swimming neurons because their ciliary beating is under bioelectric control of ion channels in the cilia. This chapter will focus on the ionic control of swimming behavior by ciliary ion channels, primarily in the holotrichous ciliate Paramecium. Voltage gated and calcium activated channels for calcium, magnesium, sodium, and potassium are regulated in a closely orchestrated manner that allows cilia to bend and propel the cell forward or backward. Sensory input that generates receptor potentials feeds into the control of this channel activity and allows the cell to turn or speed up. This in turn helps the cell to avoid predators or toxic conditions. While the focus is on P. tetraurelia and P. caudatum, the principles of ciliary ion channel activity and control are easily extendable to other ciliates and protists. The high conservation of channel and ion pump structures also extends the lessons from Paramecium to higher organisms.

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

Novel Insights into the Development and Function of Cilia Using the Advantages of the Paramecium Cell and Its Many Cilia

Paramecium species, especially P. tetraurelia and caudatum, are model organisms for modern research into the form and function of cilia. In this review, we focus on the ciliary ion channels and other transmembrane proteins that control the beat frequency and wave form of the cilium by controlling the signaling within the cilium. We put these discussions in the context of the advantages that Paramecium brings to the understanding of ciliary motility: mutants for genetic dissections of swimming behavior, electrophysiology, structural analysis, abundant cilia for biochemistry and modern proteomics, genomics and molecular biology. We review the connection between behavior and physiology, which allows the cells to broadcast the function of their ciliary channels in real time. We build a case for the important insights and advantages that this model organism continues to bring to the study of cilia.

Differential effects of Quin-2 and Quin 2-AM on ciliary responses of Paramecium caudatum at moderate calcium levels

Paramecium cells were suspended in a pH 7.1 buffered solution containing various levels of total CaCl2 (3.9 to 250 μM), Quin-2 or Quin 2-AM and in the absence or presence of various concentrations of KCl, NaCl or BaCl(2). At lower calcium levels especially, Quin-2 behaved as a potent antagonist of KCl-induced continuous ciliary reversal (50% inhibition at 4 μM) and, at higher concentrations, reduced the duration of periodic ciliary reversal responses to NaCl and BaCl(2). The efficacy of Quin-2 as an antagonist of KCl-induced ciliary reversal was reduced by elevation in extracellular calcium. Quin 2-AM, in contrast, slightly increased the duration of continuous ciliary reversal behaviors to KCl and also, in a time-dependent fashion, increased the duration of the recovery time of the cells from KCl stimulation. The results clearly indicate that any use of these indicators to measure changes in ionized calcium in this cell following cation stimulation should be limited to studies at high extracellular calcium levels - concentrations where the cells' responses to cationic stimulation are markedly attenuated.

Control of the ciliary beat by cyclic nucleotides in intact cortical sheets from Paramecium

The locomotor behavior of Paramecium depends on the ciliary beat direction and beat frequency. Changes in the ciliary beat are controlled by a signal transduction mechanism that follows changes in the membrane potential. These events take place in cilia covered with a ciliary membrane. To determine the effects of second messengers in the cilia, cortical sheets were used with intact ciliary membrane as a half-closed system in which each cilium is covered with a ciliary membrane with an opening to the cell body. Cyclic nucleotides and their derivatives applied from an opening to the cell body affected the ciliary beat. cAMP and 8-Br-cAMP increased the beat frequency and the efficiency of propulsion and acted antagonistically to the action of Ca(2+). cGMP and 8-Br-cGMP increased the efficiency of propulsion accompanying clear metachronal waves but decreased the beat frequency. These results indicate that the cyclic nucleotides affect target proteins in the ciliary axonemes surrounded by the ciliary membrane without a membrane potential and increase the efficiency of propulsion of the ciliary beat. In vitro phosphorylation of isolated ciliary axonemes in the presence of cyclic nucleotides and their derivatives revealed that the action of cAMP was correlated with the phosphorylation of 29-kDa and 65-kDa proteins and that the action of cGMP was correlated with the phosphorylation of a 42-kDa protein.

One-way calcium spill-over during signal transduction in Paramecium cells: from the cell cortex into cilia, but not in the reverse direction

We asked to what extent Ca(2+) signals in two different domains of Paramecium cells remain separated during different stimulations. Wild-type (7S) and pawn cells (strain d4-500r, without ciliary voltage-dependent Ca(2+)-channels) were stimulated for trichocyst exocytosis within 80 ms by quenched-flow preparation and analysed by energy-dispersive X-ray microanalysis (EDX), paralleled by fast confocal fluorochrome analysis. We also analysed depolarisation-dependent calcium signalling during ciliary beat rerversal, also by EDX, after 80-ms stimulation in the quenched-flow mode. EDX and fluorochrome analysis enable to register total and free intracellular calcium concentrations, [Ca] and [Ca(2+)], respectively. After exocytosis stimulation we find by both methods that the calcium signal sweeps into the basis of cilia, not only in 7S but also in pawn cells which then also perform ciliary reversal. After depolarisation we see an increase of [Ca] along cilia selectively in 7S, but not in pawn cells. Opposite to exocytosis stimulation, during depolarisation no calcium spill-over into the nearby cytosol and no exocytosis occurs. In sum, we conclude that cilia must contain a very potent Ca(2+) buffering system and that ciliary reversal induction, much more than exocytosis stimulation, involves strict microdomain regulation of Ca(2+) signals.

Immunocytochemistry of cGMP in the Cerebellum of the Immature, Adult, and Aged Rat: the Involvement of Nitric Oxide. A Micropharmacological Study

In this study we describe the localization of formaldehyde-fixed cGMP-immunoreactivity (cGMP-IR) in rat cerebellar tissue slices incubated in vitro. In the absence of phosphodiesterase inhibition, cGMP-immunofluorescence was of low intensity in tissue slices prepared from immature cerebella. Addition of isobutylmethylxanthine (IBMX) to the incubation medium resulted in the appearance of cGMP-IR in clusters of astrocytes in the internal granular layer. Addition of N-methyl-d-aspartate (NMDA), kainic acid, atrial natriuretic factor (ANF), or sodium nitroprusside (SNP) gave an intense cGMP-IR in Bergmann fibres, Bergmann cell bodies, and astrocytes in the internal granular layer. Astrocytes in the white matter showed cGMP-IR after incubation of the slice in the presence of ANF or nitroprusside, but not after NMDA or kainic acid. In addition, after SNP stimulation of cGMP production, cGMP-IR was found in fibres which were not positive for glial fibrillary acidic protein (GFAP). In the adult cerebellar slice, intense basal cGMP-immunostaining was observed in Bergmann fibres, Bergmann cell bodies, and astrocytes in the granular layer. No cGMP-IR was observed in Purkinje cells. Stimulation of the cGMP-content in the glial structures by NMDA, ANF, or SNP, was suggested by the immunocytochemical results. However, when measured biochemically, only the effect of SNP was statistically significant, and immunocytochemistry showed that SNP clearly stimulated cGMP synthesis in neuronal cell structures. In the cerebellum of the aged rat a reduced cGMP-IR was found compared to the adult, in the same structures which showed cGMP-IR in the adult. Basal cGMP-immunostaining was reduced in the presence of haemoglobin, methylene blue, by inhibiting nitric oxide synthesis with NG-monomethyl-l-arginine (NGMAr), or by depletion of external Ca2+. Also the stimulatory effect of NMDA and of ANF (partly) on the cGMP-IR was inhibited by these compounds. cGMP-IR after stimulation of guanylate cyclase by SNP was reduced by the concomitant presence of haemoglobin or methylene blue, but not by NGMAr, or by omission of Ca2+. Our results point to an important role for cGMP in the functioning of glial tissue in the cerebellum and also suggest a role for nitric oxide as an intercellular mediator in the functioning of glutamate and ANF in the cerebellum.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Paramecium--a model system for studying cellular graviperception

Experiments under varied gravitational accelerations as well as in density-adjusted media showed that sensation of gravity in protists may be linked to the known principles of mechanosensation. Paramecium, a ciliate with clear graviresponses (gravitaxis and gravikinesis) is an ideal model system to prove this hypothesis since the ciliary activity and thus the swimming behaviour is controlled by the membrane potential. It has also been assumed that the cytoplasmic mass causes a distinct stimulation of the bipolarly distributed mechano-sensitive K+ and Ca2+ ion channels in the plasma membrane in dependence of the spatial orientation of the cell. In order to prove this hypothesis, different channel blockers are currently under investigation. Gadolinium did not inhibit gravitaxis in Paramecium, showing that it does not specifically block gravireceptors. Further studies concentrated on the question of whether second messengers are involved in the gravity signal transduction chain. Exposure to 5 g for up to 10 min led to a significant increase in cAMP.

Influence of extremely low frequency electromagnetic fields on the swimming behavior of ciliates

Different species of ciliates (Paramecium biaurelia, Loxodes striatus, Tetrahymena thermophila) have been taken as model systems to study the effects of extremely low-frequency electromagnetic fields (50 Hz, 0.5-2.0 mT) on the cellular level. A dose-dependent increase in the mean swimming velocity and a decrease in the linearity of cell tracks were observed in all wild-type cells. In contrast, field-exposure did not increase the number of directional turns of the Paramecium tetraurelia pawn mutant (d4-500r), which is characterized by defective Ca2+-channels. The described changes indicate a direct effect of low frequency electromagnetic fields on the transport mechanisms of the cell membrane for ions controlling the motile activity of cilia.

A guanylyl cyclase from Paramecium with 22 transmembrane spans. Expression of the catalytic domains and formation of chimeras with the catalytic domains of mammalian adenylyl cyclases

Paramecium has a 280-kDa guanylyl cyclase. The N terminus resembles a P-type ATPase, and the C terminus is a guanylyl cyclase with the membrane topology of canonical mammalian adenylyl cyclases, yet with the cytosolic loops, C1 and C2, inverted compared with the mammalian order. We expressed in Escherichia coli the cytoplasmic domains of the protozoan guanylyl cyclase, independently and linked by a peptide, as soluble proteins. The His(6)-tagged proteins were enriched by affinity chromatography and analyzed by immunoblotting. Guanylyl cyclase activity was reconstituted upon mixing of the recombinant C1a- and C2-positioned domains and in a linked C1a-C2 construct. Adenylyl cyclase activity was minimal. The nucleotide substrate specificity was switched from GTP to ATP upon mutation of the substrate defining amino acids Glu(1681) and Ser(1748) in the C1-positioned domain to the adenylyl cyclase specific amino acids Lys and Asp. Using the C2 domains of mammalian adenylyl cyclases type II or IX and the C2-positioned domain from the Paramecium guanylyl cyclase we reconstituted a soluble, all C2 adenylyl cyclase. All enzymes containing protozoan domains were not affected by Galpha(s)/GTP or forskolin, and P site inhibitors were only slightly effective.

Graviorientation in protists and plants

Gravitaxis, gravikinesis, and gravitropism are different graviresponses found in protists and plants. The phenomena have been intensively studied under variable stimulations ranging from microgravity to hypergravity. A huge amount of information is now available, e.g. about the time course of these events, their adaptation capacity, thresholds, and interaction between gravity and other environmental stimuli. There is growing evidence that a pure physical mechanism can be excluded for orientation of protists in the gravity field. Similarly, a physiological signal transduction chain has been postulated in plants. Current investigations focus on the question whether gravity is perceived by intracellular gravireceptors (e.g. the Muller organelle of the ciliate Loxodes, barium sulfate vacuoles in Chara rhizoids or starch statoliths in higher plants) or whether the whole cell acts as a sedimenting body exerting pressure on the lower membrane. Behavioral studies in density adjusted media, effects of inhibitors of mechano-sensitive ion channels or manipulations of the proposed gravireceptor structures revealed that both mechanisms have been developed in protists and plants. The threshold values for graviresponses indicate that even 10% of the normal gravitational field can be detected, which demands a focusing and amplifying system such as the cytoskeleton and second messengers.

Protein substrates for cGMP-dependent protein phosphorylation in cilia of wild type and atalanta mutants of Paramecium

In the ciliated protozoan Paramecium, swimming direction is regulated by voltage-gated Ca2+ channels in the ciliary membrane. In response to depolarizing stimuli, intraciliary Ca2+ rises, triggering reversal of the ciliary power stroke and backward swimming. One class of Ca(2+)-unresponsive behavioral mutants of Paramecium, atalanta mutants, cannot swim backward even though they have functional Ca2+ channels in their ciliary membrane. Several atalanta mutants were characterized with regard to several Ca(2+)-dependent activities, but no significant difference between wild type and the mutants was detected. However, one allelic group, atalanta A (initially characterized by Hinrichsen and Kung [1984: Genet. Res. Camb. 43:11-20]), showed a helical swimming path of opposite handedness from that of wild-type cells when detergent-permeabilized cells ("models") were reactivated with MgATP. When cGMP-dependent protein kinase purified from wild-type cells was added to atalanta A models, the handedness of the swimming path was reversed. Cyclic GMP stimulated in vitro phosphorylation of several proteins in isolated cilia, and the pattern of phosphoproteins was very similar for wild type and atalanta mutants, with one exception: a protein of 59 kDa was phosphorylated much less in the mutant ata A. When ciliary proteins were separated by gel electrophoresis and then phosphorylated "on blot" by purified cGMP-dependent protein kinase, phosphoprotein patterns were similar in wild type and ata mutants except that a 48 kDa protein (p48) from ata A3 was more heavily phosphorylated. This difference in p48 phosphorylation was also observed with cGMP-dependent protein kinase purified from ata A3 mutant cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel touch-induced, Ca(2+)-dependent phobic response in a flagellate green alga

The biflagellate green alga Spermatozopsis similis exhibits a remarkable avoidance reaction in addition to the photophobic or stop response characteristic of such algae. S. similis normally swims forward with its anteriorly attached flagella directed posteriorly and propagating sine-like waves base to tip. Upon contact with surfaces or other cells, S. similis responds with rapid backward swimming, covering distances of up to 50 microns in 140 to 220 msec. This reaction, which we term the mechanoshock response, also can be triggered by vigorous mechanical stimulation, but not by physiological light intensities. It consists of 3 phases: (1) a rapid acceleration phase with average duration of 31 msec; (2) a phase of about 66 msec with constant high speed (maximal velocities of > 600 microns.sec-1) or slow deceleration; and (3) a deceleration phase of approximately 83 msec, followed by a stop or short period of circling. The cells then resume forward swimming in a random direction. Prior to the mechanoshock response the flagella rapidly are brought together into a close parallel configuration extending anteriorly of the cell body. They then appear to propel the cell by undulatory beating, while the cell describes a pronounced helical path. Small decreases in the extracellular Ca2+ concentration, as well as low concentrations of Ba2+, strongly suppress the probability of this phobic reaction. We conclude that this mechanoshock response involves large Ca2+ influxes, probably mediated by mechanosensitive and/or stretch-activated ion-channel(s).

A rapid calcium influx during exocytosis in Paramecium cells is followed by a rise in cyclic GMP within 1 s

The synchrony of trichocyst exocytosis in Paramecium allows temporal correlation of associated events. Using quenched flow we observed a Ca2+ influx concurrent with exocytosis within 80 ms after stimulation with the secretagogue aminoethyldextran. Cyclic AMP did not change in depency of stimulation. Cyclic GMP transiently increased after 500 ms, culminating at 2 s, and thus considerably lags behind exocytosis induction and influx of Ca2+. Both Ca2+ influx and rise in cGMP are known to be induceable also by Ba2+ or veratridine, allegedly via the opening of ciliary Ca2+ channels. However, only veratridine stimulated exocytosis. We conclude that both aminoethyldextran and veratridine induce an exocytosis-associated Ca2+ influx, which is responsible for the rise in cGMP, through an as yet unknown pathway.

On the stimulation of soluble and particulate guanylate cyclase in the rat brain and the involvement of nitric oxide as studied by cGMP immunocytochemistry

The localization of the particulate and soluble guanylate cyclase in the rat brain was studied using cGMP-immunocytochemistry. The cGMP was fixed to tissue protein using a formaldehyde fixative, and an antibody against cGMP was used which was raised against a cGMP-formaldehyde-thyroglobulin conjugate. We used the atrial natriuretic factor (ANF) as a model compound to stimulate the particulate enzyme and sodium nitroprusside (SNP) to stimulate the soluble enzyme. Sequential immunostaining for cGMP and glial fibrillary acidic protein (GFAP) showed that the great majority of the ANF-responsive, cGMP-producing cells were astrocytes. These ANF-responsive cells were found in discrete parts of the CNS; not all astrocytes in these regions were responsive to ANF. SNP stimulated cGMP in abundantly present neuronal fibres throughout the CNS; few neuronal cell bodies showed increased cGMP production after SNP. Moreover, SNP also raised cGMP in astrocytes, however, not all astrocytes showed the response to SNP. These results suggest that cells might be present in the CNS which contain both the soluble and the particulate guanylate cyclase. It was demonstrated that in the immature cerebellum, the cGMP was raised in glial structures in response to N-methyl-D-aspartate (NMDA), ANF, SNP, and kainic acid. The response to NMDA and kainic acid was sensitive to inhibition of the nitric oxide synthesis from L-arginine by NG-methyl-L-arginine. Surprisingly the response to ANF localized in the molecular layer and the granular layer was also sensitive to inhibition by NG-methyl-L-arginine, whereas the response to ANF in the deep nuclei was not. A small depolarization induced by 10 to 20 mmol/l K+ induced an increase in cGMP in chopped hippocampus tissue which showed a biphasic temporal characteristic. The initial, fast (30 sec), peak was shown to be localized in varicose fibres throughout the hippocampus, whereas the slower response (10 min) was localized in astrocytes. These studies demonstrate that the different enzymes which synthesize cGMP are differently localized. However, there is also a time dependency in the activation of the guanylate cyclases, which becomes apparent in different structures at different times. The possible role of cGMP as a regulator of ion homeostase is discussed.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

In vitro phosphorylation of Paramecium axonemes and permeabilized cells

This study seeks to identify phosphoproteins in axonemes from Paramecium tetraurelia whose phosphorylation responses to adenosine 3', 5'-cyclic monophosphate (cAMP) and Ca2+ parallel responses induced by these agents in ciliary behavior in this cell. In purified axonemes, over 15 bands ranging from Mr greater than 300 kDa to 19 kDa on SDS-PAGE incorporate 32P from adenosine 5'-gamma-[32P]triphosphate (gamma-32P-ATP) at pCa 7 in the absence of cAMP. A major band whose label turns over rapidly was identified at Mr 43 kDa. In the presence of 5 microM cAMP, more than eight bands, but not the Mr 43 kDa band, were labeled additionally or enhanced their labeling. These phosphoproteins and their kinases are structural components of the axoneme. Overall, some of the same major bands are labeled in the presence of cAMP in Triton X-100-permeabilized paramecia that retain their behavioral responses and in axonemes mechanically isolated from these cells. In particular, two major bands have been identified whose phosphorylation is greatly enhanced by cAMP at low concentrations: 1) a 29 kDa polypeptide whose cAMP-dependent phosphorylation is diminished at pCa 4 compared with pCa 7 and 2) a 65 kDa polypeptide whose phosphorylation is pCa insensitive. These polypeptides meet minimal criteria for signal-sensitive regulators of motility parameters in the Paramecium axoneme.

Regulation of axonemal Mg2+-ATPase from Paramecium cilia: effects of Ca2+ and cyclic nucleotides

Ciliary activity is regulated by Ca2+ and cyclic nucleotides, but the molecular mechanisms of the regulation are unknown. We have tested the ability of Ca2+ and cyclic nucleotides to alter ciliary Mg2+-ATPase or to stimulate phosphorylation of axonemal dynein. Mg2+-ATPase activity in cilia and axonemes from Paramecium was stimulated 2-fold by micromolar Ca2+, but this Ca2+ sensitivity was lost upon solubilization of the dyneins from the axoneme. The Ca2+-sensitive component of ciliary Mg2+-ATPase activity was inhibited by the dynein inhibitors vanadate and Zn2+, but was insensitive to the calmodulin antagonists calmidazolium and melittin. Dynein activity in the high-salt extract from axonemes was also insensitive to calmidazolium. Calmodulin did not sediment with 22 S or 12 S dyneins on sucrose gradients containing Ca2+, but it did sediment in the region from 19 S to 14 S. Mg2+-ATPase activity in ciliary fractions was unaltered in the presence of cAMP or cGMP. However, polypeptides associated with the 22 S and 12 S dyneins, as well as proteins of 19 S, 15 S, and 8 S, were substrates for endogenous ciliary kinases. High molecular weight polypeptides that sedimented at 22 S and 19 S were phosphorylated in a cyclic nucleotide-stimulated manner.

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

cGmp synthesis in cultured cerebellar neurons is stimulated by glutamate via a Ca2+-mediated, differentiation-dependent mechanism

Sensitivity of cyclic GMP synthesis to stimulation by excitatory amino acids, depolarizing agents, and divalent cation ionophores develops during the differentiation of cerebellar neurons in culture; in each case calcium influx appears responsible for activating guanylate cyclase. The developmental event being followed is not the appearance of either the soluble or the particulate form of the enzyme since both are present throughout. The possible role of a differentiation-dependent calcium-modulating protein is discussed.