Light-dependent GTP-binding proteins in squid photoreceptors

The ordered visual transduction complex of the squid photoreceptor membrane

The study of visual transduction has given invaluable insight into the mechanisms of signal transduction by heptahelical receptors that act via guanine nucleotide binding proteins (G-proteins). However, the cyclic-GMP second messenger system seen in vertebrate photoreceptor cells is not widely used in other cell types. In contrast, the retina of higher invertebrates, such as squid, offers an equally accessible transduction system, which uses the widespread second messenger chemistry of an increase in cytosolic calcium caused by the production of inositol-(1,4,5)-trisphosphate (InsP3) by the enzyme phospholipase C, and which may be a model for store-operated calcium influx. In this article, we highlight some key aspects of invertebrate visual transduction as elucidated from the combination of biochemical techniques applied to cephalopods, genetic techniques applied to flies, and electrophysiology applied to the horseshoe crab. We discuss the importance and applicability of ideas drawn from these model systems to the understanding of some general processes in signal transduction, such as the integration of the cytoskeleton into the signal transduction process and the possible modes of regulation of store-operated calcium influx.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

Immunological demonstration of Gq-protein in Limulus photoreceptors

The phototransduction cascade in invertebrates involves the coupling of rhodopsin activation to the action of the enzyme phospholipase C. This step is performed by G-proteins. An antibody against the alpha-subunit of a mouse Gq type G-protein recognized protein bands in Western blots of lateral eye and ventral nerve photoreceptors of Limulus. The protein bands had an apparent molecular mass of about 42 kDa. The antibody also recognized protein bands of a similar molecular mass in immunoblots of brain and intestine tissue. Immunoreactivity was found in lateral eye frozen sections where it was confined to the rhabdom region. When the antibody was applied to ultrathin sections of ventral nerve photoreceptors, the highest density of labeling was found on the rhabdomeral microvilli, but gold particles were also scattered in the cytoplasm. We conclude that a G-protein of the type Gq participates in the phototransduction of Limulus.

Interaction of guanosine 5'-triphosphate binding protein Gq from Sepia officinalis with illuminated rhodopsin bound to concanavalin A

The heterotrimeric guanosine 5'-triphosphate (GTP)-binding protein Gq was suggested to couple the light receptor rhodopsin with the effector phospholipase C in visual cells of invertebrates. We indirectly linked Gq from Sepia officinalis to a concanavalin A-sepharose column via rhodopsin. Rhodopsin had been solubilized previously with 10 mM n-dodecyl-beta-maltoside from the purified photosensory membrane under illumination. All three subunits of the Gq were released almost pure by elution with 100 microM GTP. The alpha and beta subunits were identified by specific antipeptide antisera. The alpha subunit has a relative molecular mass of 46 kDa, and the beta subunit of 35 kDa. The gamma subunit corresponds to a 9 kDa polypeptide owing to the molecular mass, which is similar to the G gamma subunit of squid. The use of specific antibodies shows that neither actin nor G-protein related to transducin were in the fractions eluted with GTP or alpha-methyl mannoside. We demonstrate that all three subunits of Gq were associated with rhodopsin of invertebrates. Such use of a lectin column might be useful for further investigations of the interaction of rhodopsin and Gq.

Immunoelectron-microscopic study of G-protein distribution in photoreceptor cells of the cephalopod Sepia officinalis

Previous studies suggest that more than one single light-stimulated transduction pathway seems to be present in photoreceptor cells of invertebrates. Accordingly, more than one light-dependent G-protein has been detected in squid photoreceptor cells. Two different antibodies were used to locate the distribution of G-proteins in Sepia photoreceptors. One antiserum (anti-G alpha-common) has been raised against the peptide CGAGESGKSTIVKQMK. This sequence is found in most G alpha-protein subunits, it is also present in transducin of vertebrates. This sequence however, is conserved only partially in G alpha q from photosensory membranes of the squid. The second antiserum (anti-G alpha q) has been raised against the peptide QLNLKEYNLV. This sequence is present at the C-terminus of e.g. G alpha q from mouse brain and is identical also in squid. Anti-G alpha q very strongly interacted with the rhabdomeres of the photoreceptor cells from Sepia. In the non-rhabdomeric cytoplasm of the photoreceptor cells gold granules (bound to the second antibody) were almost absent. With anti-G alpha-common, only a small number of gold particles could be found at the rhabdomeres. The non-rhabdomeric cytoplasm was not labelled. Thus, further G-proteins that might be involved in a light dependent phototransduction mechanism besides the phosphoinositide pathway can hardly be expected to belong to the group of 'common' G-proteins.

Purification, characterization, and partial amino acid sequence of a G protein-activated phospholipase C from squid photoreceptors

Invertebrate visual transduction is thought to be initiated by photoactivation of rhodopsin and its subsequent interaction with a guanyl nucleotide-binding protein (G protein). The identities of the G protein and its target effector have remained elusive, although evidence suggests the involvement of a phospholipase C (PLC). We have identified a phosphatidylinositol-specific PLC from the cytosol of squid retina. The enzyme was purified to near-homogeneity by a combination of carboxymethyl-Sepharose and heparin-Sepharose chromatography. The purified PLC, identified as an approximately 140-kDa protein by sodium dodecyl sulfate-polyacrylamide gels, hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) at a rate of 10-15 mumol/min/mg of protein with 1 microM Ca2+. The partial amino acid sequence of the protein showed homology with a PLC cloned from a Drosophila head library (PLC21) and lesser homology with Drosophila norpA protein and mammalian PLC beta isozymes. Reconstitution of purified squid PLC with an AlF(-)-activated 44-kDa G protein alpha subunit extracted from squid photoreceptor membranes resulted in a significant increase in PIP2 hydrolysis over a range of Ca2+ concentrations while reconstitution with mammalian Gt alpha or Gi 1 alpha was without effect. These results suggest that cephalopod phototransduction is mediated by G alpha-44 activation of a 140-kDa cytosolic PLC.

Concentrations of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate within the distal segment of squid photoreceptors

Although inositol trisphosphate (InsP3) is a key substance in phototransduction of invertebrate photoreceptors, its intracellular concentration remains unknown. The purpose of this study was to assay its concentration and the concentration of its precursor, phosphatidylinositol bisphosphate (PtdInsP2), within squid photoreceptors. Rhabdomeric membranes were purified and their PtdInsP2 content measured with a phosphate assay after the extracted phospholipids were deacylated and separated by ion-exchange chromatography. At least 75% of the total PtdInsP2 found in the retinal homogenate was associated with the plasma membranes of the rhabdomeric microvilli, where PtdInsP2 was 3.1 +/- 0.7% of the total phospholipids, a level comparable to values published for rat brain. In terms of rhodopsin, microvillar membranes contained 3.7 +/- 0.9 mol PtdInsP2/mol rho. The InsP3 content of living retinas was measured with a radioreceptor assay. The basal content of dark-adapted retinas was 0.15 +/- 0.05 InsP3/rho, equivalent to 30 +/- 9 nmol/g tissue that is about twice that of rat brains. Flash illumination (approximately 1 ms in duration) that photoactivated 1% of rhodopsin increased the level about fivefold to 0.68 +/- 0.22 InsP3/rho. Corresponding decrease in PtdInsP2 was undetectable as it was within measurement errors. For PtdInsP2, the measured content corresponds to 5.6 +/- 1.4 mM within the volume of rhabdomere. Maximal light-induced concentration of InsP3 is calculated to be 1.2 +/- 0.4 mM within the cytoplasm of the distal segment. Each photoactivated rhodopsin leads to the formation of < or = 500 InsP3 molecules when measured 15 s after the flash.(ABSTRACT TRUNCATED AT 250 WORDS)

The molecular cloning of the squid (Loligo forbesi) visual Gq-alpha subunit and its expression in Saccharomyces cerevisiae

The sequence of the alpha-subunit of the major G-protein from the squid (Loligo forbesi) retina was predicted from its cDNA to be a member of the Gq subclass. The abundance of the squid Gq-alpha in the squid photoreceptor membranes suggests that the protein functions in phototransduction; the sequence of this G-protein is consistent with it mediating the light-dependent activation of a phospholipase C. The squid G-alpha was expressed in the yeast Saccharomyces cerevisiae, where it was unable to replace the function of GPA1, the yeast G-alpha homologue that regulates the mating response, suggesting that Gq-alpha was unable to interact with the endogenous G-beta gamma (STE4-STE18).

ATP-independent deactivation of squid rhodopsin

Deactivation of light-activated squid rhodopsin was studied in vitro using GTP gamma S binding by G-protein as a direct measure of rhodopsin activity. Deactivation was inhibited by dilution of the retinal suspension or by removal of soluble components. Deactivation could be restored by addition of soluble material to washed membranes. These results indicate that the deactivation is not due entirely to a conformational transition within rhodopsin itself, but depends on the interaction with other molecules. The possibility that phosphorylation is involved in the deactivation was studied. Deactivation occurred in the presence and absence of added ATP. Deactivation also occurred in the presence of kinase inhibitors and after addition of apyrase, which reduced residual ATP levels to below 1 microM. These results indicate that light-induced phosphorylation is not required for deactivation of squid rhodopsin. In this regard deactivation of squid rhodopsin is different from that of vertebrate rhodopsin, which requires phosphorylation.

Activation of the GTP-binding protein Gq by rhodopsin in squid photoreceptors

Photoaffinity labelling by a GTP analogue has been used to identify a 42 kDa band as the major G alpha subunit in squid photoreceptor membranes, recently identified by partial sequence analysis to be a member of the Gq sub-group of GTP-binding proteins [Pottinger, Ryba, Keen & Findlay (1991) Biochem. J. 279, 323-326]. Guanine-nucleotide-binding displacement analysis gave a stoichiometry of 1 G-protein per 12.5 rhodopsin molecules, the same as in vertebrate rod photoreceptors. Binding was not detected above background in the dark, but was rapidly activated by light. Unlike vertebrate transducin, this G-protein is very temperature-sensitive. GTP binding is maximal at temperatures less than 10 degrees C and is much decreased after several minutes above 18 degrees C. The light-stimulated GTPase rate is maximal around 10 degrees C, above which the loss of binding sites counteracts the increase in hydrolytic rate per site. Earlier studies described light-sensitive G alpha components of 40 and 45 kDa, by ADP-ribosylation in the presence of cholera and pertussis toxins. These are now shown to be very minor components, as the prolonged treatment at elevated temperature required for ADP-ribosylation is sufficient to inactivate the major G alpha totally. Unlike the minor G alpha components, the 42 kDa G alpha is not inhibited by Ca2+.

Three components in the light-induced current of the Limulus ventral photoreceptor

1. Light-induced currents were measured in Limulus ventral nerve photoreceptors using a two-electrode voltage clamp. Three kinetically distinct components in the light-induced current could be distinguished by varying the light adaptation state of the photoreceptor and the intensity of the stimulus light. 2. The components could be partly separated by choosing appropriate stimulus intensities and dark adaptation time. Thus the properties of the components could be separately studied. The first component is the first to recover after a light adaptation, appears temporally first in the light-induced response, has the lowest activation threshold and is the smallest. The second component needs a longer time to recover after an adapting illumination and its kinetics differ from that of the other components. Applying a bright stimulus to a dark-adapted cell a third component can be observed late in the response. 3. The time to peak of the first and the third components depended on the stimulus intensity, but not on the dark adaptation time. The time to peak of the second component became shorter the longer the dark adaptation time. For a constant adaptation state the time to the maximum of component 2 was independent, but those of components 1 and 3 were dependent on the membrane voltage. 4. To exclude the possibility of the contribution of voltage-gated currents, light-activated currents were measured at clamp potentials more negative than -50 mV after adding 4-aminopyridine into the bath solution or injecting tetraethyl-ammonium chloride into the cell. The properties of the three components remained unchanged under these conditions. 5. The I-V curve of the first component was flat at negative membrane potentials and had a strong outward rectification at positive membrane potentials. The I-V curve of component 3 showed a negative resistance at potentials more negative than about -30 mV. In contrast, the I-V curve for the second component was always nearly linear. 6. No membrane potential was found where the light-induced current was zero. Instead, current traces close to the reversal potential showed a complex waveform indicating different reversal potentials for the three components. 7. The results indicate that the current components are caused by three different populations of light-sensitive channels. The different activations, deactivations and recovery kinetics of the components suggest that the three types of channels are activated by distinct intracellular transmitters.

Localization of phototransduction in Limulus ventral photoreceptors: a demonstration using cell-free rhabdomeric vesicles

Second messengers are involved in a number of cellular responses to a variety of stimuli. Diffusion of these second messengers likely will determine the speed and efficiency of such responses. Localization, particularly in large cells, would enhance the efficiency of such transduction systems by restricting the volume in which this diffusion takes place and thereby limiting the diffusion of soluble messengers. Phototransduction in Limulus ventral photoreceptors involves second-messenger systems; the volume of this cell is quite large, but the effect of a single photoexcited rhodopsin molecule is exerted over light-dependent channels localized within a very small area of the plasma membrane. In order to investigate localization of phototransduction in these photoreceptors, we have compared the light responses of small vesicles (photoballs) excised from these cells with those of the intact photoreceptors. We found that the basic kinetics of excitation and adaptation of the photoballs are essentially identical to those of the intact cell. This indicates that all of the necessary machinery for phototransduction is present and intact in the photoball and that any diffusion of second messengers that affect the normal light response of the cell must occur within a region at least as small as our photoballs (on the order of 1 micron3).

Light-induced GTPase activity and GTP[gamma S] binding in squid retinal photoreceptors

Illumination greatly increases the GTPase activity in homogenates of squid (Loligo) whole retinas or rhabdomeric membranes. Adenylylimidodiphosphate inhibits the light-insensitive (but not the light-sensitive) GTPase activity in these homogenates. Illumination also greatly increases the binding of GTP[gamma S] to the rhabdomeric membranes. This binding at saturating illuminations indicates that there are approximately 10-100 times more rhodopsin molecules than G-protein molecules in squid photoreceptors. Each light-activated rhodopsin molecule activates about 10 G-protein molecules which might provide amplification for the first stage of the phototransduction cascade.

The identification and purification of the heterotrimeric GTP-binding protein from squid (Loligo forbesi) photoreceptors

The principal GTP-binding protein (G-protein) from squid (Loligo forbesi) photoreceptor membranes has been identified by amino acid sequencing. The heterotrimeric protein was purified by detergent solubilization and ion-exchange chromatography. The amino acid sequence of the G-protein alpha-subunit (G-alpha) indicates that this subunit is closely related to the recently characterized Gq subgroup, whereas the G-gamma subunit varies widely in sequence from other homologues.

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)