Bistability in spinal motoneurons in vivo: systematic variations in rhythmic firing patterns

In the presence of the monoamines serotonin and norepinephrine, spinal motoneurons can exhibit bistable behavior, in which a brief period of excitatory input evokes prolonged self-sustained firing. A brief inhibitory input returns the cell to the quiescent state. To determine whether motoneurons differ in their capacity for bistable behavior, intracellular recordings were obtained in the decerebrate cat preparation. To enhance the likelihood of encountering bistable behavior, the noradrenergic alpha1 agonist methoxamine was applied to the ventral surface of the cord. The capacity of the cells to produce bistable behavior was assessed from the duration of self-sustained firing evoked by a brief (1.5 s) excitatory synaptic input from muscle spindle Ia afferents. About 35% (17 of 49) of the cells produced steady self-sustained firing for >3 s and were considered fully bistable. The other 32 cells ( approximately 65%) were partially bistable, with self-sustained firing lasting <3 s. Fully bistable cells tended to have lower current thresholds for spike initiation and slower axonal conduction velocities than did partially bistable cells. This suggests that fully bistable motoneurons innervate fatigue resistant muscle fibers. The frequency-current (F-I) relations of the motoneurons were characterized with slow triangular current ramps. Fully bistable cells displayed an acceleration in firing rate immediately on initiation of rhythmic firing. The F-I gain after completion of the acceleration was positive. Fully bistable cells also displayed a hysteresis in the current level for firing threshold with the ascending threshold occurring at substantially higher current level than the descending one. Additionally, these current thresholds usually were centered about zero current, so that the ascending current threshold was positive while the descending current threshold was negative. This negative offset meant that fully bistable cells could exhibit tonic firing without depolarizing injected current. Partially bistable cells exhibited very different F-I characteristics. Firing rate acceleration was just as large as in fully bistable cells but did not occur until well above the current level needed to initiate rhythmic firing. F-I gain after acceleration was negative, there was little to no hysteresis between the ascending and descending firing thresholds, and both thresholds were above the zero current level. These properties of partially bistable cells suggest their functional role is in tasks requiring relatively brief, high forces. The low thresholds of fully bistable cells mean they will be readily recruited in low force tasks like posture, where their prolonged self-sustained firing would be advantageous.

Bistability in spinal motoneurons in vivo: systematic variations in persistent inward currents

Bistable behavior in spinal motoneurons consists of self-sustained firing evoked by a brief period of input. However, not all motoneurons possess an equal capacity for bistable behavior. In the companion paper, we found that self-sustained firing was persistent for long periods only in motoneurons with low rheobases and slow axonal conduction velocities. High rheobase, fast conduction velocity motoneurons tend to be only partially bistable in that self-sustained firing lasts at most 1-2 s. The mechanisms underlying these differences between fully and partially bistable motoneurons were investigated by measuring their current voltage (I-V) relationships in the decerebrate cat preparation after administration of the noradrenergic alpha1 agonist methoxamine. Slow (8 mV/s) triangular voltage commands were applied using the discontinuous single-electrode voltage-clamp technique. Both fully and partially bistable cells exhibited a region of negative I-V slope due to activation of a strong, persistent inward current. The peak amplitude of the total persistent inward current (IPIC) was equally large in fully and partially bistable cells, but there were substantial differences in how IPIC was activated and deactivated. In fully bistable cells, the offset of IPIC on the descending phase of the triangular voltage command occurred at a substantially more hyperpolarized voltage then its onset on the rising phase. Thus the I-V function of fully bistable cells exhibited marked hysteresis. Partially bistable cells had significantly less hysteresis. The lack of hysteresis in partially bistable cells was due to a greater decay of IPIC with time than that seen in fully bistable cells. Furthermore, the range over which activation and deactivation of IPIC occurred was more depolarized in partially than in fully bistable cells. The I-V functions were compared with frequency-current (F-I) functions from the same cells, the characteristics of which were presented in the companion paper. The strong onset-offset difference in IPIC in fully bistable cells corresponded to a similarly large hysteresis for the thresholds of their F-I functions. The reduced onset-offset difference for IPIC in partially bistable cells corresponded to a lack of hysteresis in F-I thresholds. Thus the properties of IPIC accounted for the main differences in the F-I behavior seen between fully and partially bistable cells.

Gene structure and chromosome localization of the G gamma c subunit of human cone G-protein (GNGT2)

Phototransduction in the vertebrate rod and cone photoreceptors is regulated by structurally homologous and yet distinct groups of signaling proteins. We have previously identified in bovine retinas a cone-specific G-protein gamma subunit (G gamma c, previously named G gamma b), which may play a key role in coupling the cone visual pigment to phosphodiesterase (O. C. Ong et al., 1995, J. Biol. Chem. 270:8495-8500). We report here the characterization of human G gamma c and its gene structure. Human G gamma c subunit shares a high degree of sequence identity with the corresponding bovine G gamma c isoform (85%) and human rod G gamma 1 (63%). The protein is specifically localized in cones, as indicated by immunohistochemical staining using anti-G gamma c antibodies. Nucleotide sequence analysis of the G gamma c gene (GNGT2) reveals a structure consisting of three exons and two introns, with the intron splice sites similar to that of the rod G gamma 1 gene (GNGT1). By using fluorescence in situ hybridization, we have further localized the human GNGT2 gene to chromosome 17q21. The elucidation of the G gamma c gene structure would facilitate the identification of genetic defects associated with cone degeneration.

Patterns of facial laceration from blunt trauma

The clinical characteristics of craniofacial injuries resulting from various modes of blunt trauma, including motor vehicle accidents, falls, and assault, have been described extensively in the literature. In this study, specifically targeted blunt trauma to selected areas of the face was used to recreate soft-tissue laceration injuries on 19 cadaver heads. The patterns of laceration produced were then examined by location, size, penetrated skin depth, and associated muscle and bony involvement. Results showed reproducible patterns of lacerations on the forehead, bilateral superior orbital rim, nose, perimaxillary region, and chin. Six of 19 cadaver faces were undermined prior to blunt trauma to determine the effects of subcutaneous attachments on laceration patterns. Results showed no consistent difference in laceration patterns between undermined skin and intact skin. Our findings suggest that in response to blunt trauma, the skin breaks along selected lines of least resistance that closely parallel cleavage lines of the face and that the patterns of laceration generated are associated with inherent structural and biomechanical properties of the skin, independent of subcutaneous attachments. Blunt trauma lacerations to the face therefore may occur in a consistent and reproducible manner and may be subject to classification.

Phosducin and betagamma-transducin interaction I: effects of post-translational modifications

The interaction between phosducin and betagamma-transducin plays regulatory roles in light adaptation of photoreceptors. Both phosducin and betagamma-transducin undergo post-translational modifications, with phosducin modified by phosphorylation and the gamma subunit of betagamma-transducin by farnesylation and carboxylmethylation. In this study we exploited the electrophoretic mobilities of these native proteins to develop a micro binding assay and examined the effects of post-translational modifications on binding affinities. It was found that decarboxylmethylation of gamma-transducin increased the mobility of betagamma-transducin during native gel electrophoresis, but decreased the apparent affinity for phosducin by about 2-fold. Phosphorylation of phosducin by protein kinase A increased the mobility but decreased the apparent affinity for betagamma-transducin by at least 3-fold.

Influence of voltage-sensitive dendritic conductances on bistable firing and effective synaptic current in cat spinal motoneurons in vivo

1. After application of the noradrenergic alpha 1 agonist methoxamine, muscle spindle Ia input evoked bistable firing patterns (i.e., persistent discharges after Ia input ceased) in adult spinal motoneurons in the decerebrate cat preparation. These bistable discharge patterns were compared with the Ia currents generated in voltage-clamp conditions. 2. During voltage clamp at depolarized holding potentials, the Ia effective synaptic current underwent strong amplification. In those cells with strong bistable firing, a prolonged tail current followed the Ia input. Because the voltage clamp held the behavior of somatic conductances constant, these data indicate that voltage-sensitive conductances in the motoneuron dendrites made an important contribution to bistable firing.

Nutritional regulation of late meiotic events in Saccharomyces cerevisiae through a pathway distinct from initiation

The IME1 gene is essential for initiation of meiosis in the yeast Saccharomyces cerevisiae, although it is not required for growth. Here we report that in stationary-phase cultures containing low concentration of glucose, cells overexpressing IME1 undergo the early meiotic events, including DNA replication, commitment to recombination, and synaptonemal complex formation and dissolution. In contrast, later meiotic events, such as chromosome segregation, commitment to meiosis, and spore formation, do not occur. Thus, nutrients can repress the late stages of meiosis independently of their block of initiation. Cells arrested at this midpoint in meiosis are relatively stable and can resume meiotic differentiation if transferred to sporulation conditions. Resumption of meiosis does not require repression of IME1 expression, since IME1 RNA levels stay high after transfer of the arrested cells to sporulation medium. These results suggest that meiosis in S. cerevisiae is a paradigm of a differentiation pathway regulated by signal transduction at both early and late stages.

Restoration of extensor excitability in the acute spinal cat by the 5-HT2 agonist DOI

1. The decerebrate cat preparation with an intact spinal cord is characterized by a high degree of excitability in extensor motoneuron pools, which is eliminated by acute spinalization. Subtype-specific agonists for serotonin (5-HT) were investigated in terms of their effectiveness in restoring the extensor excitability following spinalization. 2. Our hypothesis was that 5-HT2 receptors have the primary role in enhancement of extensor reflex excitability, whereas 5-HT1A and 5-HT1B/D receptors are relatively unimportant. Reflex excitability was assessed from the tonic levels of force and electromyographic (EMG) output from the ankle extensors medial gastrocnemius (MG) and soleus (SOL), and from the reflex forces in both these muscles generated by ramp-and-hold stretches of MG. 3. Before spinal transection, MG and SOL usually exhibited a small amount of tonic background EMG activity and force output. Ramp-and-hold stretch of MG generated a large-amplitude reflex response. Spinal transection at the level of T10 virtually abolished tonic background activity in both extensors and greatly attenuated the MG stretch reflex. Ventral topical application of the selective 5-HT2A/2C agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane hydrochloride (DOI) restored the amplitude of the MG stretch reflex in a dose-dependent fashion. However, a considerable portion of the DOI-mediated restoration of MG stretch reflex force was due to elevation of tonic background force levels above previous intact cord levels. 4. The DOI-induced increase in extensor tonic background excitability and facilitation of MG stretch reflex were reversed by ventral topical administration of the selective 5-HT2 antagonist ketanserin. No increase in extensor excitability was observed in spinalized preparations after administration of either the 5-HT1A agonist (+-)-8-hydroxy-dipropylaminotetralin hydrobromide or the 5-HT1B/1D agonist 7-trifluoromethyl-4-(4 methyl-1-piperazinyl)-pyrrolo[1,2- a]quinoxaline maleate. These data strongly suggest that the DOI-induced facilitation of extensor stretch reflex and tonic activity in spinalized preparations is mediated through an action on spinal 5-HT2 receptors. 5. One important difference between the actions of DOI in spinalized versus intact states was that the DOI-induced tonic and reflex forces in the spinalized state were subject to irregular oscillations. In contrast, DOI did not noticeably affect the smoothness of reflex force generation in the intact state. This discrepancy was probably due to the effects of clasp knife inhibition from muscular free nerve endings, which have potent reflex actions in the spinalized but not intact states. Thus DOI elevated excitability levels but did not alter the effects of spinalization on stretch reflex patterns.

Adenylyl cyclase integrates multiple G protein signals to modulate calcium currents in neonatal rabbit heart

We investigated the effects of added beta gamma subunits of G proteins (G beta gamma) on beta-adrenergic responsiveness of transmembrane Ca2+ currents (ICa) in ventricular myocytes from neonatal rabbits. G beta 1 gamma 1 purified from retinal rods was dialyzed into cells via the voltage clamp micro-electrode. Stimulation of ICa by isoproterenol was not affected by added intracellular G beta 1 gamma 1 or by carbachol alone but was completely blocked by combined G beta 1 gamma 1 and carbachol. Pretreatment of cells with pertussis toxin or temporal separation of carbachol and isoproterenol allowed stimulation of ICa by isoproterenol in cells dialyzed with G beta 1 gamma 1. Carbachol and G beta 1 gamma 1 together also did not prevent stimulation of ICa by dibutyryl-cyclic AMP. Thus, rather than simply inactivating Gs alpha by mass action, G beta 1 gamma 1 acts in concert with carbachol to inhibit isoproterenol stimulation of ICa.

Molecular cloning and characterization of the G protein gamma subunit of cone photoreceptors

The phototransduction process in cones has been proposed to involve a G protein that couples the signal from light-activated visual pigment to the effector cyclic GMP phosphodiesterase. Previously, we have identified and purified a G beta gamma complex composed of a G beta 3 isoform and an immunochemically distinct G gamma subunit (G gamma 8) from bovine retinal cones (Fung, B. K.-K., Lieberman, B. S., and Lee, R. H. (1992) J. Biol. Chem. 267, 24782-24788; Lee, R. H., Lieberman, B.S., Yamane, H. K., Bok, D., and Fung, B. K.-K. (1992a) J. Biol. Chem. 267, 24776-24781). Based on the partial amino acid sequence of this cone G gamma 8, we screened a bovine retinal cDNA library and isolated a cDNA clone encoding G gamma 8. The cDNA insert of this clone includes an open reading frame of 207 bases encoding a 69-amino acid protein. The predicted protein sequence of G gamma 8 shares a high degree of sequence identity (68%) with the G gamma (G gamma 1) subunit of rod transducin. Similar to rod G gamma 1, it terminates in a CIIS motif that is the site for post-translational modification by farnesylation. Messenger RNA for G gamma 8 is present at a high level in the retina and at a very low level in the lung, but is undetectable in other tissues. Immunostaining of bovine retinal sections with an antipeptide antibody against the N-terminal region of G gamma 8 further shows a differential localization of G gamma 8 to cones with a pattern indistinguishable from that of G beta 3. This finding suggests that G beta 3 gamma 8 is a component of cone transducin involved in cone phototransduction and color vision.

Inhibitors of protein tyrosine kinase pp60v-src: sterol sulfates from the brittle star Ophiarachna incrassata

Bioactivity-guided fractionation of the extracts of the green brittle star Ophiarachna incrassata using a protein tyrosine kinase pp60v-src inhibition assay led to the isolation of a new sterol sulfate [2] together with four known ones (1,3-5). All five compounds were found to inhibit protein tyrosine kinase pp 60v-src.

The marine natural product, halistanol trisulfate, inhibits pp60v-src protein tyrosine kinase activity

Halistanol trisulfate, a sulfated steroid derivative, was isolated from the extracts of two different marine sponges (genus Topsentia) by bioassay-guided fractionation. It exhibited an IC50 of approximately 4 microM against pp60v-src, the oncogenic protein tyrosine kinase encoded by Rous sarcoma virus. Removing the sulfate groups by acid hydrolysis produced the inactive tris-alcohol, halistanol. The kinetics of inhibition were examined and revealed that halistanol trisulfate is a competitive inhibitor with respect to the peptide substrate, [val5]-angiotensin II, and a mixed inhibitor with respect to ATP. A number of monosulfated steroids were studied for protein tyrosine kinase inhibitory activity, but were found to be inactive.

Muscarinic K+ channels are activated by beta gamma subunits and inhibited by the GDP-bound form of alpha subunit of transducin

It was reported that transducin beta gamma subunits (T beta gamma) inhibited cardiac muscarinic K+ (KACh) channels activated through GK. This result has been used to support the proposal that GK activates this channel through its alpha subunits. Here, we have reexamined the effects of transducin subunits on KACh channels. T beta gamma applied to the internal side of the inside-out patch membrane activated KACh channels in a concentration-dependent manner (EC50 approximately 1 microM). In contrast, the GDP-bound form of transducin alpha subunits (50 nM-3 microM), but not T beta gamma (1 nM-1 microM), irreversibly inhibited KACh channel openings induced either by GTP plus acetylcholine or by GTP gamma S (guanosine 5'-3-O-(thio)triphosphate). This inhibition was antagonized by exogenously applied T beta gamma (600 nM-3 microM) or brain G protein beta gamma subunits (10-30 nM). These data, opposite to the previous report, indicate that GK activates KACh channels through its beta gamma subunits.

Structure of malhamensilipin A, an inhibitor of protein tyrosine kinase, from the cultured chrysophyte Poterioochromonas malhamensis

A new chlorosulfolipid, malhamensilpin A [1] was isolated from the cultured chrysophyte Poterioochromonas malhamensis. Malhamensilipin A was demonstrated to be a modest inhibitor of pp60v-src protein tyrosine kinase. The structure was determined by detailed spectral analysis to be a novel C24 hexachloro lipid containing a vinyl sulfate ester (2,11,12,13,15,16-hexachloro-14-hydroxy-n-tetracos-1E-enol-1-sulfa te).

New cycloartanol sulfates from the alga Tydemania expeditionis: inhibitors of the protein tyrosine kinase pp60v-src

Bioactivity-directed fractionation of the extracts of the green alga Tydemania expeditionis using the protein tyrosine kinase pp60v-src led to the isolation of three new cycloartanol disulfates, 1-3, which show modest inhibition of this enzyme. The structures were deduced by spectroscopic methods.

Assignment of the phosducin (PDC) gene to human chromosome 1q25-1q32.1 by somatic cell hybridization and in situ hybridization

Phosducin is a soluble photoreceptor phosphoprotein that probably modulates phototransduction in the retina and thus qualifies as a potential candidate gene for retinitis pigmentosa. Using both human/mouse somatic cell hybrids and in situ hybridization to human metaphase chromosomes, we have mapped this gene to chromosome 1q25-1q32.1.

Expression of conformationally constrained adhesion peptide in an antibody CDR loop and inhibition of natural killer cell cytotoxic activity by an antibody antigenized with the RGD motif

We report that an antibody engineered to express three Arg-Gly-Asp (RGD) repeats in the third complementarity-determining region of the heavy chain (antigenized antibody) efficiently inhibits the lysis of human erythroleukemia K-562 cells by natural killer (NK) cells. Synthetic peptides containing RGD did not inhibit. Inhibition was specific for the (RGD)3-containing loop and required simultaneous occupancy of the Fc receptor (CD16) on effector cells. The antigenized antibody inhibited other forms of cytotoxicity mediated by NK cells but not cytotoxicity mediated by major histocompatibility complex-restricted cytotoxic T lymphocytes (CTL). A three-dimensional model of the engineered antibody loop shows the structure and physicochemical characteristics probably required for the ligand activity. The results indicate that an RGD motif is involved in the productive interaction between NK and target cells. Moreover, they show that peptide expression in the hypervariable loops of an antibody molecule is an efficient procedure for stabilizing oligopeptides within a limited spectrum of tertiary structures. This is a new approach towards imparting ligand properties to antibody molecules and can be used to study the biological function and specificity of short peptide motifs, including those involved in cell adhesion.

Oocyte maturation in starfish is mediated by the beta gamma-subunit complex of a G-protein

The stimulation of meiotic maturation of starfish oocytes by the hormone 1-methyladenine is mimicked by injection of beta gamma subunits of G-proteins from either retina or brain. Conversely, the hormone response is inhibited by injection of the GDP-bound forms of alpha i1 or alpha t subunits, or by injection of phosducin; all of these proteins should bind free beta gamma. alpha-subunit forms with reduced affinity for beta gamma (alpha i1 or alpha t bound to hydrolysis-resistant GTP analogs, or alpha i1-GMPPCP treated with trypsin to remove the amino terminus of the protein) are less effective inhibitors of 1-methyladenine action. These results indicate that the beta gamma subunit of a G-protein mediates 1-methyladenine stimulation of oocyte maturation.

Irish setter dogs affected with rod/cone dysplasia contain a nonsense mutation in the rod cGMP phosphodiesterase beta-subunit gene

Irish setter dogs affected with a rod/cone dysplasia (locus designation, rcd1) display markedly elevated levels of retinal cGMP during postnatal development. The photoreceptor degeneration commences approximately 25 days after birth and culminates at about 1 year when the population of rods and cones is depleted. A histone-sensitive retinal cGMP phosphodiesterase (PDE; EC 3.1.4.35) activity, a marker for photoreceptor PDEs, was shown previously to be present in retinal homogenates of immature, affected Irish setters. Here we report that, as judged by HPLC separation, this activity originates exclusively from cone photoreceptors, whereas rod PDE activity is absent. An immunoreactive product the size of the PDE alpha subunit, but none the size of the beta subunit, can be detected on immunoblots of retinal extracts of affected dogs, suggesting a null mutation in the PDE beta-subunit gene. Using PCR amplification of Irish setter retinal cDNA, we determined the complete coding sequence of the PDE beta subunit in heterozygous and affected animals. The affected PDE beta-subunit mRNA contained a nonsense amber mutation at codon 807 (a G-->A transition converting TGG to TAG), which was confirmed to be present in putative exon 21 of the affected beta-subunit gene. The premature stop codon truncates the beta subunit by 49 residues, thus removing the C-terminal domain that is required for posttranslational processing and membrane association. These results suggest that the rcd1 gene encodes the rod photoreceptor PDE beta subunit and that a nonsense mutation in this gene is responsible for the production of a nonfunctional rod PDE and the photoreceptor degeneration in the rcd1/rcd1 Irish setter dogs.

Regulation of retinal cGMP cascade by phosducin in bovine rod photoreceptor cells. Interaction of phosducin and transducin

Photoexcitation of retinal rod photoreceptor cells involves the activation of cGMP enzyme cascade in which sequential activation of rhodopsin, transducin, and the cGMP phosphodiesterase in the rod outer segment constitutes the signal amplification mechanism. Phosducin, a 33-kDa phosphoprotein, has been shown to form a tight complex with the T beta gamma subunit of transducin. In this study, we examined the interaction of phosducin-T beta gamma and the possible regulatory role of phosducin on the cGMP cascade. Addition of phosducin to photolyzed rod outer segment (ROS) membrane reduced the GTP hydrolysis activity of transducin as well as the subsequent activation of the cGMP phosphodiesterase. Phosducin also inhibited the pertussis toxin-catalyzed ADP-ribosylation of transducin, indicating that the interaction between the T alpha and T beta gamma subunits of transducin was interrupted upon binding of phosducin. The inhibitory effects of phosducin were reversed by the addition of exogenous T beta gamma. These results suggest that phosducin is capable of regulating the amount of T beta gamma available to interact with T alpha to form the active transducin complex and thereby functions as a negative regulator of the cGMP cascade. The phosducin-induced alteration of the subunit organization of transducin was examined by chemical cross-linking method using para-phenyl dimaleimide as cross-linker. It was found that the cross-linking among T alpha and T beta gamma was blocked in the presence of phosducin. This result implies that T beta gamma may undergo a conformational change upon phosducin binding which leads to the release of T alpha. Since phosducin is a soluble protein, the interaction with transducin only occurs when transducin is dissociated from ROS disc membrane. Indeed, phosducin failed to dissociate membrane-bound transducin and did not inhibit the initial cycle of transducin activation as measured by the presteady state GTP hydrolysis. However, phosducin interacts effectively with transducin released into solution after the initial activation and blocks the re-binding of T alpha. T beta gamma to ROS membrane by forming a tight complex with T beta gamma. This interaction may play an important role in regulating the turnover of the cGMP cascade in photoreceptor cells.

A third form of the G protein beta subunit. 2. Purification and biochemical properties

Visual excitation in cones is thought to involve a cone-specific G protein (cone transducin) that transduces the light signal detected by the cone visual pigment into an increase in the enzymatic activity of a cGMP phosphodiesterase. In the preceding paper, we have shown that the G beta 3 isoform of G proteins is specifically localized in bovine cone photoreceptors and proposed that it might be a component of cone transducin. We reported here the purification from bovine retinal extract of a cone-specific T beta 3 gamma complex (where T is transducin), which is composed of a G beta 3 subunit and an immunochemically distinct G gamma subunit. Our purification of this complex is based on a two-stage procedure; the first stage consists of a series of column chromatographies that yield a mixture of purified T beta gamma substantially enriched in T beta 3 gamma, and the second stage involves the removal of all of the rod-specific T beta 1 gamma from the mixture using an affinity column of immobilized monoclonal antibodies directed against the rod T gamma subunit of transducin. Using this procedure, we were able to obtain sufficient amounts of T beta 1 gamma and T beta 3 gamma to begin a comparative study of their properties. We showed that T beta 3 gamma is distinguishable from T beta 1 gamma by isoelectric focusing under nondenaturing conditions. The G beta 3 polypeptide of T beta 3 gamma also migrates slightly slower than the G beta 1 polypeptide of T beta 1 gamma on denaturing polyacrylamide gels. Analysis of the interactions of T beta 3 gamma with other retinal proteins indicated that it has a lower affinity for the T alpha subunit of rod transducin but appears to complex with a phosducin-like protein. The differences in the intrinsic biochemical properties of T beta 3 gamma as compared to T beta 1 gamma may partially account for the lower light sensitivity of cones.

A third form of the G protein beta subunit. 1. Immunochemical identification and localization to cone photoreceptors

Vertebrate retinal cones play a major role in both photopic vision and color perception. Although the molecular mechanism of visual excitation in the cone is not as well understood as in the rod, it is generally thought to involve a cone-specific G protein (cone transducin) that couples the cone visual pigment to a cGMP phosphodiesterase. Like all other G proteins, cone transducin is most likely a heterotrimer consisting of G alpha, G beta, and G gamma subunits. A G alpha subunit of cone transducin has been localized to the outer segment of bovine cones, but its associated G beta and G gamma subunits are unknown. To identify the G beta subunit involved in the phototransduction process of cones, we have developed a panel of antipeptide antisera against the most diverse region of the amino acid sequences encoded by G beta 1, G beta 2, and G beta 3 cDNAs and used them to determine the distribution of the G beta isoforms in different retinal preparations. We found that the G beta 3 subunit is present in bovine retinal transducin and phosducin-T beta gamma complex preparations which were previously thought to contain only G beta 1. Analysis of its subcellular distribution indicated that G beta 3 is predominantly cytoplasmic. Immunocytochemical staining of bovine retinal sections with the anti-G beta 3 antiserum further revealed a specific localization of G beta 3 in cones but not in rods. In contrast, anti-G beta 1 antiserum stained only the rods. These results suggest that G beta 3 is the G beta subunit of cone transducin and confirms the proposition that rods and cones utilize distinct signaling proteins for phototransduction.

Marine sponge polyketide inhibitors of protein tyrosine kinase

The marine polyketide natural product, halenaquinone, was shown to be an irreversible inhibitor of pp60v-src, the oncogenic protein tyrosine kinase encoded by the Rous sarcoma virus. This compound had an IC50 of approximately 1.5 microM against pp60v-src and also inhibited the ligand-stimulated kinase activity of the human epidermal growth factor receptor with an IC50 of approximately 19 microM. Halenaquinone blocked the proliferation of a number of cultured cell lines, including several transformed by oncogenic protein tyrosine kinases. Halenaquinol, xestoquinone, halenaquinol sulfate, and several simple synthetic quinone analogs were also shown to inhibit pp60v-src.