A mutation that increases a novel calcium-activated potassium conductance of Paramecium tetraurelia

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Isolation and characterization of magbane, a magnesium-lethal mutant of paramecium

Discerning the mechanisms responsible for membrane excitation and ionic control in Paramecium has been facilitated by the availability of genetic mutants that are defective in these pathways. Such mutants typically are selected on the basis of behavioral anomalies or resistance to ions. There have been few attempts to isolate ion-sensitive strains, despite the insights that might be gained from studies of their phenotypes. Here, we report isolation of "magbane," an ion-sensitive strain that is susceptible to Mg2+. Whereas the wild type tolerated the addition of > or =20 mm MgCl2 to the culture medium before growth was slowed and ultimately suppressed (at >40 mm), mgx mutation slowed growth at 10 mm. Genetic analysis indicated that the phenotype resulted from a recessive single-gene mutation that had not been described previously. We additionally noted that a mutant that was well described previously (restless) is also highly sensitive to Mg2+. This mutant is characterized by an inability to control membrane potential when extracellular K+ concentrations are lowered, due to inappropriate regulation of a Ca2+-dependent K+ current. However, comparing the mgx and rst mutant phenotypes suggested that two independent mechanisms might be responsible for their Mg2+ lethality. The possibility that mgx mutation may adversely affect a transporter that is required for maintaining low intracellular Mg2+ is considered.

Bistable membrane potential of the ciliate Coleps hirtus

In normal recording solution, the swimming pattern of the freshwater ciliate Coleps hirtus, belonging to the class Prostomatea, consists of alternating periods of nearly linear forward swimming and circular swimming within a small area. Current-clamp recordings were performed to elucidate the mechanism for this behaviour. No members of this class have previously been studied using electrophysiological techniques. The ciliates were maintained in culture and fed on the planctonic alga Rhodomonas minuta. The membrane potential showed spontaneous shifts between a more negative (deep) level of approximately −50 mV and a less negative (shallow) level of approximately −30 mV. The input resistance and capacitance at the more negative level were approximately 400 M capomega and 120 pF respectively. C. hirtus displayed a pronounced inward rectification, which was virtually insensitive to 1 mmol l(−1) Cs(+) and almost completely blocked by 1 mmol l(−1) Ba(2+). Depolarising current injections failed to evoke graded, regenerative Ca(2+) spikes. However, current-induced depolarisations from the more negative potential level (−50 mV) showed a pronounced shoulder during the repolarising phase. Increased current injections prolonged the shoulder, which occasionally stabilised at the shallow membrane potential (−30 mV). The membrane potential could be shifted to the deep level by brief hyperpolarising current injections. Similar biphasic membrane properties have not been reported previously in any ciliate. The bistability of the membrane potential was abolished in Ca(2+)-free solution containing Co(2+) or Mg(2+). In Ca(2+)-free solution containing 1 mmol l(−1) Ba(2+), brief depolarising current injections at the deep potential level evoked all-or-nothing action potentials with a prolonged plateau coinciding with the shallow potential. We conclude that the deep membrane potential in C. hirtus corresponds to the traditional resting potential, whereas the shallow level is a Ca(2+)-dependent plateau potential. In normal solution, the direction of the ciliary beat was backwards at the deep potential level and forwards at the shallow membrane potential, probably reflecting the two main phases of the swimming pattern.

Correlation between loss of a Mg2+ conductance and an adaptation defect in a mutant of Paramecium tetraurelia

Paramecium tetraurelia responds to chronic KCl-induced depolarization by swimming backward, but the ciliate recovers within seconds and then undergoes a prolonged adaptation period during which sensitivity to external stimuli is altered radically. We examined the role of Mg2+ in this phenomenon, prompted by finding that mutations in the eccentric-A gene both suppressed a Mg(2+)-specific conductance and prevented adaptation. Adaptation of the wild type proceeded normally when extracellular Mg2+ was varied from 0-20 mM, however, suggesting that channel-mediated Mg2+ fluxes were not involved. In seeking alternative explanations for the eccentric mutant phenotype, we ascertained that there was an osmotic component to adaptation but that K(+)-induced depolarization was the primary stimulus. We also noted that wild-type and eccentric mutant cells depolarized by equivalent amounts in KCl, suggesting that the genetic lesion must lie downstream of membrane-potential change. We also examined whether the adaptation-induced behavioral changes and, indeed, the defect in eccentric might be explained in terms of Mg2+ and Na+ efflux during behavioral testing, but experimental observations failed to support this notion. Finally, we consider the possibility that eccentric gene mutation prevents adaptation by interfering with intracellular free Mg2+ homeostasis in Paramecium.

In vivo Paramecium mutants show that calmodulin orchestrates membrane responses to stimuli

Paramecium generates a Ca2+ action potential and can be considered a one-cell animal. Rises in internal [Ca2+] open membrane channels that specifically pass K+, or Na+. Mutational and patch-clamp studies showed that these channels, like enzymes, are activated by Ca(2+)-calmodulin. Viable CaM mutants of Paramecium have altered transmembrane currents and easily recognizable eccentricities in their swimming behavior, i.e. in their responses to ionic, chemical, heat, or touch stimuli. Their CaMs have amino-acid substitutions in either C- or N-terminal lobes but not the central helix. Surprisingly, these mutations naturally fall into two classes: C-lobe mutants (S101F, I136T, M145V) have little or no Ca(2+)-dependent K+ currents and thus over-react to stimuli. N-lobe mutants (E54K, G40E+D50N, V35I+D50N) have little or no Ca(2+)-dependent Na+ current and thus under-react to certain stimuli. Each mutation also has pleiotropic effects on other ion currents. These results suggest a bipartite separation of CaM functions, a separation consistent with the recent studies of Ca(2+)-ATPase by Kosk-Kosicka et al. [41, 55]. It appears that a major function of Ca(2+)-calmodulin in vivo is to orchestrate enzymes and channels, at or near the plasma membrane. The orchestrated actions of these effectors are not for vegetative growth at steady state but for transient responses to stimuli epitomized by those of electrically excitable cells.

Genetic dissection of Ca2(+)-dependent ion channel function in Paramecium

The ciliated protozoan, Paramecium, broadcasts the activity of its individual ion channel classes through its swimming behaviour. This fact has made it possible to isolate mutants with defective ion currents, simply by selecting individuals with abnormal swimming patterns. At least four of Paramecium's ion currents are activated by rising intracellular calcium concentration, including two K+ currents and a Na+ current. A variety of cell lines with defects in these Ca2(+)-dependent currents have been isolated: in several cases, the defects have been traced to mutations in the structural gene for calmodulin. Sequence analysis of calmodulins from these and other Ca2(+)-dependent ion-current mutants may enable a detailed mapping of putative channel interaction domains on the surface of the calmodulin molecule.

Evidence for two K+ currents activated upon hyperpolarization of Paramecium tetraurelia

Hyperpolarization of voltage-clamped Paramecium tetraurelia in K+ solutions elicits a complex of Ca2+ and K+ currents. The tail current that accompanies a return to holding potential (-40 mV) contains two K+ components. The tail current elicited by a step to -110 mV of greater than or equal to 50-msec duration contains fast-decaying (tau approximately 3.5 msec) and slow-decaying (tau approximately 20 msec) components. The reversal potential of both components shifts by 55-57 mV/10-fold change in external [K+], suggesting that they represent pure K+ currents. The dependence of the relative amplitudes of the two tail currents on duration of hyperpolarization suggests that the slow K+ current activates slowly and is sustained, whereas the fast current activates rapidly during hyperpolarization and then rapidly inactivates. Iontophoretic injection of a Ca2+ chelator, EGTA, specifically reduces slow tail-current amplitude without affecting the fast tail component. Both K+ currents are inhibited by extracellular TEA+ in a concentration-dependent, noncooperative manner, whereas the fast K+ current alone is inhibited by 0.7 mM quinidine.

Interactions between mutants with defects in two Ca2(+)-dependent K+ currents of Paramecium tetraurelia

Paramecium tetraurelia possesses two Ca2(+)-dependent K+ currents, activated upon depolarization IK(Ca,d), or upon hyperpolarization IK(Ca,h). The two currents are mediated by pharmacologically distinct ion channel populations. Three mutations of P. tetraurelia affect these currents. Pantophobiac A mutations (pntA) cause calmodulin sequence defects, resulting in the loss of both Ca2(+)-dependent K+ currents. A second mutation, TEA-insensitive A (teaA), greatly enhances IK(Ca,d) but has no affect on IK(Ca,h). A third mutation, restless (rst), also increases IK(Ca,d) slightly, but its principle effect is in causing an early activation of IK(Ca,h). Interactions between the products of these three genes were investigated by constructing three double mutants. Both teaA and rst restore IK(Ca,d) and IK(Ca,h) in pantophobiac A1, but the phenotypes of teaA and rst are not corrected by a second mutation. These observations may indicate a role for the gene products of teaA and rst in regulating the activity of IK(Ca,d) and IK(Ca,h), respectively.

Calcium-dependent potassium channel in Paramecium studied under patch clamp

We have studied a class of Ca2+i-dependent K channels in inside-out excised membrane patches from Paramecium under patch clamp. single channels had a conductance of 72 +/- 9.0 pS in a solution containing 100 mM K+. The channels were selective for K+ over Rb+ with the permeability ratio of 1: 0.56, and over Na+, Cs+ or NH+4 with a ratio 1: less than 0.1. The channel activity was dependent on Ca2+i, which was applied to the cytoplasmic side; the Ca2+i concentration for the half maximal activation was 2 microM. The Hill coefficient for the Ca2+i dependence of the channel activity was 2.58, indicating that more than two Ca2+i bindings are necessary for full activation. Unlike most Ca2+i-dependent K channels in other organisms, the channels in Paramecium were slightly more active upon hyperpolarization than upon depolarization. The voltage dependence was fitted to a Boltzmann curve with 41.2 mV per e-fold change in channel activity. While a high Ca2+i concentration activated the channels, it also irreversibly reduced the channel activity over time. The decay of channel activity occurred faster at higher Ca2+i concentrations. Quaternary ammonium ions suppressed ion passage through the channel; more highly alkylated quaternary ammonium ions were more efficient in blocking. Ba2+i and Ca2+i were relatively ineffective in blockage. it was concluded that these Ca2+i-dependent K channels in Paramecium are different from the previously described Ca2+i-dependent K channels, and are perhaps of a novel class.

L-glutamate-induced membrane hyperpolarization and behavioural responses in Paramecium tetraurelia

Paramecium tetraurelia is attracted to L-glutamic acid concentrations of 10(-9) M to 10(-4) M in a behavioural assay. Electrophysiological studies show that P. tetaurelia responds to L-glutamate application with hyperpolarization. This response is transient, even in the continued presence of the stimulus. The concentration dependence of the membrane potential response is similar to that of the behavioural responses, although the threshold concentration of L-glutamate required for hyperpolarization is three orders of magnitude lower than for attraction. The membrane potential response to L-glutamate persists following artificial deciliation of P. tetraurelia. While application of L-glutamate to P. tetraurelia invariably elicits a hyperpolarization, withdrawal of the stimulus frequently results in a second transient membrane response, in the form of either a hyperpolarization or a depolarization. It is suggested that these 'off-responses' may have a significant role in maintaining a behavioural response to L-glutamate.

Neomycin inhibits the calcium current of Paramecium

The duration of high-K+ stimulated backward swimming is a commonly used bioassay for estimating the amplitude of the inward calcium current of Paramecium. Electrophysiological analysis confirmed that concentrations of neomycin which decreased the duration of stimulated backward swimming also reduced the isolated inward calcium current. Other polycations were also effective in this bioassay and their effectiveness was correlated with the number of their positive charges. Paramecium is therefore a convenient model system for studying the effects of compounds such as neomycin on calcium currents as well as their mechanisms of action.

A calcium-dependent potassium current is increased by a single-gene mutation in Paramecium

The membrane currents of wild type Paramecium tetraurelia and the behavioral mutant teaA were analyzed under voltage clamp. The teaA mutant was shown to have a greatly increased outward current which was blocked completely by the combined use of internally delivered Cs+ and external TEA+. This, along with previous work (Satow, Y., Kung, C., 1976, J. Exp. Biol. 65:51-63) identified this as a K+ current. It was further found to be a calcium-activated K+ current since this increased outward K+ current cannot be elicited when the internal calcium is buffered with injected EGTA. The mutation pwB, which blocks the inward calcium current, also blocks this increased outward K+ current in teaA. This shows that this mutant current is activated by calcium through the normal depolarization-sensitive calcium channel. While tail current decay kinetic analysis showed that the apparent inactivation rates for this calcium-dependent K+ current are the same for mutant and wild type, the teaA current activates extremely rapidly. It is fully activated within 2 msec. This early activation of such a large outward current causes a characteristic reduction in the amplitude of the action potential of the teaA mutant. The teaA mutation had no effect on any of the other electrophysiological parameters examined. The phenotype of the teaA mutant is therefore a general decrease in responsiveness to depolarizing stimuli because of a rapidly activating calcium-dependent K+ current which prematurely repolarizes the action potential.