Inhibitors of asparagine-linked oligosaccharide processing alter the kinetics of the nicotinic acetylcholine receptor

Deglycosylation of Shaker KV channels affects voltage sensing and the open-closed transition

Voltage-gated ion channels are subject to posttranslational modification, including glycosylation. Lopez-Rodriguez and Holmgren show that, in Shaker K_V channels, deglycosylation influences voltage sensing and open–closed transitions but not binding of ligands to the protein.

Most membrane proteins are subject to posttranslational glycosylation, which influences protein function, folding, solubility, stability, and trafficking. This modification has been proposed to protect proteins from proteolysis and modify protein–protein interactions. Voltage-activated ion channels are heavily glycosylated, which can result in up to 30% of the mature molecular mass being contributed by glycans. Normally, the functional consequences of glycosylation are assessed by comparing the function of fully glycosylated proteins with those in which glycosylation sites have been mutated or by expressing proteins in model cells lacking glycosylation enzymes. Here, we study the functional consequences of deglycosylation by PNGase F within the same population of voltage-activated potassium (K_V) channels. We find that removal of sugar moieties has a small, but direct, influence on the voltage-sensing properties and final opening–closing transition of Shaker K_V channels. Yet, we observe that the interactions of various ligands with different domains of the protein are not affected by deglycosylation. These results imply that the sugar mass attached to the voltage sensor neither represents a cargo for the dynamics of this domain nor imposes obstacles to the access of interacting molecules.

Influence of the NR3A subunit on NMDA receptor functions

Various combinations of subunits assemble to form the NMDA-type glutamate receptor (NMDAR), generating diversity in its functions. Here we review roles of the unique NMDAR subunit, NR3A, which acts in a dominant-negative manner to suppress receptor activity. NR3A-containing NMDARs display striking regional and temporal expression specificity, and, unlike most other NMDAR subtypes, they have a low conductance, are only modestly permeable to Ca(2+), and pass current at hyperpolarized potentials in the presence of magnesium. While glutamate activates triheteromeric NMDARs composed of NR1/NR2/NR3A subunits, glycine is sufficient to activate diheteromeric NR1/NR3A-containing receptors. NR3A dysfunction may contribute to neurological disorders involving NMDARs, and the subunit offers an attractive therapeutic target given its distinct pharmacological and structural properties.

The Kv1.2 potassium channel: the position of an N-glycan on the extracellular linkers affects its protein expression and function

Voltage-gated potassium Kv1 channels have three extracellular linkers, the S1-S2, the S3-S4, and the S5-P. The S1-S2 is the only linker that has an N-glycan and it is at a conserved position on this linker on Kv1.1-Kv1.5 and Kv1.7 channels. We hypothesize that an N-glycan is found at only this position due to its effect on folding, trafficking, and/or function of these channels. To investigate this hypothesis, N-glycosylation sites were engineered at different positions on the extracellular linkers of Kv1.2 to determine the effects of N-glycans on channel surface protein expression and function. Our data suggest that for Kv1 channels, (1) placing an N-glycan at non-native positions on the S1-S2 linker decreased cell surface protein expression but the N-glycan still affected function similarly as if it were at its native position, (2) placing a non-native N-glycan on the S3-S4 linker significantly altered function, and (3) placing a non-native N-glycan on the S5-P linker disrupted both trafficking and function. We suggest that Kv1 channels have an N-glycan at a conserved position on only the S1-S2 linker to overcome the constraints for proper folding, trafficking, and function that appear to occur if the N-glycan is moved from this position.

The glycosylation state of Kv1.2 potassium channels affects trafficking, gating, and simulated action potentials

We presented evidence previously that decreasing the glycosylation state of the Kv1.1 potassium channel modified its gating by a combined surface potential and a cooperative subunit interaction mechanism and these effects modified simulated action potentials. Here we continued to test the hypothesis that glycosylation affects channel function in a predictable fashion by increasing and decreasing the glycosylation state of Kv1.2 channels. Compared with Kv1.2, increasing the glycosylation state shifted the V(1/2) negatively with a steeper G-V slope, increased activation kinetics with little change in deactivation kinetics or in their voltage-dependence, and decreased the apparent level of C-type inactivation. Decreasing the glycosylation state had essentially the opposite effects and shifted the V(1/2) positively with a shallower G-V slope, decreased activation kinetics (and voltage-dependence), decreased deactivation kinetics, and increased the apparent level of C-type inactivation. Single channel conductance was not affected by the different glycosylation states of Kv1.2 tested here. Hyperpolarized or depolarized shifts in V(1/2) from wild type were apparently due to an increased or decreased level of channel sialylation, respectively. Data and modeling suggested that the changes in activation properties were mostly predictable within and between channels and were consistent with a surface potential mechanism, but those on deactivation properties were not predictable and were more consistent with a conformational mechanism. Moreover the effect on the deactivation process appeared to be channel-type dependent as well as glycosylation-site dependent. The glycosylation state of Kv1.2 also affected action potentials in simulations. In addition, preventing N-glycosylation decreased cell surface Kv1.2 expression levels by approximately 40% primarily by increasing partial endoplasmic reticulum retention and this effect was completely rescued by Kv1.4 subunits, which are glycosylated, but not by cytoplasmic Kvbeta2.1 subunits. The nonglycosylated Kv1.2 protein had a similar protein half-life as the glycosylated protein and appeared to be folded properly. Thus altering the native Kv1.2 glycosylation state affected its trafficking, gating, and simulated action potentials. Differential glycosylation of ion channels could be used by excitable cells to modify cell signaling.

Effects of Kv1.1 channel glycosylation on C-type inactivation and simulated action potentials

Kv1.1 channels are brain glycoproteins that play an important role in repolarization of action potentials. In previous work, we showed that lack of N-glycosylation, particularly lack of sialylation, of Kv1.1 affected its macroscopic gating properties and slowed activation and C-type inactivation kinetics and produced a depolarized shift in the steady-state activation curve. In our current study, we used single channel analysis to investigate voltage-independent C-type inactivation in both Kv1.1 and Kv1.1N207Q, a glycosylation mutant. Both channels underwent brief and long-lived closures, and the lifetime and frequency of the long-lived closed states were voltage-independent and similar for both channels. We found that, as in macroscopic measurements, Kv1.1N207Q exhibited a approximately 8 mV positive shift in its single channel fractional open time (fo) and a shallower fo-voltage slope compared with Kv1.1. Data suggested that C-type inactivation reflected the equilibration time with at least two slow voltage-independent long-lived closed states that followed the rapid activation process. In addition, data simulation indicated that the C-type inactivation process reflected the equilibration time between the open state and at least two long-lived closed states. Moreover, the faster macroscopic current decay in Kv1.1 mostly reflected a slower equilibration time in these channels as compared with Kv1.1N207Q. Finally, action potential simulations indicated that the N207Q mutation broaden the action potential and decreased the interspike interval. The shape of the action potential was not significantly affected by C-type inactivation, however, for a given channel, C-type inactivation increased the interspike interval. Data and simulations suggested that excitable cells could use differences in K(+) channel glycosylation degree as an additional mechanism to increase channel functional diversity which could modify cell excitability.

Glycosylation affects rat Kv1.1 potassium channel gating by a combined surface potential and cooperative subunit interaction mechanism

The effect of glycosylation on Kv1.l potassium channel function was investigated in mammalian cells stably transfected with Kv1.l or Kv1.1N207Q. Macroscopic current analysis showed that both channels were expressed but Kv1.1N207Q, which was not glycosylated, displayed functional differences compared with wild-type, including slowed activation kinetics, a positively shifted V 1/2, a shallower slope for the conductance versus voltage relationship, slowed C-type inactivation kinetics, and a reduced extent of and recovery from C-type inactivation. Kv1. 1N207Q activation properties were also less sensitive to divalent cations compared with those of Kv1.l. These effects were largely due to the lack of trans-Golgi added sugars, such as galactose and sialic acid, to the N207 carbohydrate tree. No apparent change in ionic current deactivation kinetics was detected inKv1.1N207Q compared with wild-type. Our data, coupled with modelling, suggested that removal of the N207 carbohydrate tree had two major effects. The first effect slowed the concerted channel transition from the last dosed state to the open state without changing the voltage dependence of its kinetics. This effect contributed to the G-V curve depolarization shift and together with the lower sensitivity to divalent cations suggested that the carbohydrate tree and its negatively charged sialic acids affected the negative surface charge density on the channel's extracellular face that was sensed by the activation gating machinery. The second effect reduced a cooperativity factor that slowed the transition from the open state to the dosed state without changing its voltage dependence. This effect accounted for the shallower G-V slope, and contributed to the depolarized G-V shift, and together with the inactivation changes it suggested that the carbohydrate tree also affected channel conformations. Thus N-glycosylation, and particularly terminal sialylation, affected Kv1.l gating properties both by altering the surface potential sensed by the channel's activation gating machinery and by modifying conformational changes regulating cooperative subunit interactions during activation and inactivation. Differences in glycosylation pattern among closely related channels may contribute to their functional differences and affect their physiological roles.

Determinants involved in Kv1 potassium channel folding in the endoplasmic reticulum, glycosylation in the Golgi, and cell surface expression

Kv1.1 and Kv1.4 potassium channels are expressed as mature glycosylated proteins in brain, whereas they exhibited striking differences in degree of trans-Golgi glycosylation conversion and high cell surface expression when they were transiently expressed as homomers in cell lines. Kv1.4 exhibited a 70% trans-Golgi glycosylation conversion, whereas Kv1.1 showed none, and Kv1.4 exhibited a approximately 20-fold higher cell surface expression level as compared with Kv1.1. Chimeras between Kv1.4 and Kv1.1 and site-directed mutants were constructed to identify amino acid determinants that affected these processes. Truncating the cytoplasmic C terminus of Kv1.4 inhibited its trans-Golgi glycosylation and high cell surface expression (as shown by Li, D., Takimoto, K., and Levitan, E. S. (2000) J. Biol. Chem. 275, 11597-11602), whereas truncating this region on Kv1.1 did not affect either of these events, indicating that its C terminus is not a negative determinant for these processes. Exchanging the C terminus between these channels showed that there are other regions of the protein that exert a positive or negative effect on these processes. Chimeric constructs between Kv1.4 and Kv1.1 identified their outer pore regions as major positive and negative determinants, respectively, for both trans-Golgi glycosylation and cell surface expression. Site-directed mutagenesis identified a number of amino acids in the pore region that are involved in these processes. These data suggest that there are multiple positive and negative determinants on both Kv1.4 and Kv1.1 that affect channel folding, trans-Golgi glycosylation conversion, and cell surface expression.

Glycosylation of GIRK1 at Asn119 and ROMK1 at Asn117 has different consequences in potassium channel function

GIRK (G protein-gated inward rectifier K(+) channel) proteins play critical functional roles in heart and brain physiology. Using antibodies directed to either GIRK1 or GIRK4, site-directed mutagenesis, and specific glycosidases, we have investigated the effects of glycosylation in the biosynthesis and heteromerization of these proteins expressed in oocytes. Both GIRK1 and GIRK4 have one extracellular consensus N-glycosylation site. Using chimeras between GIRK1 and GIRK4 as well as a GIRK1 N-glycosylation mutant, we report that GIRK1 was glycosylated at Asn(119), whereas GIRK4 was not glycosylated at Asn(132). GIRK1 membrane-spanning domain 1 was required for optimal glycosylation at Asn(119) because a chimera that contained GIRK4 membrane-spanning domain 1 significantly reduced the addition of a carbohydrate structure at this site. This finding may partly account for the reason that GIRK4 is not glycosylated at Asn(132), either as a homomer or when coexpressed with GIRK1. When the GIRK1(N119Q) mutant was coexpressed with GIRK4, the biophysical properties of the heteromeric channel and the magnitude of the agonist-induced currents were similar to those of controls. Thus, N-glycosylation of GIRK1 at Asn(119) does not appear to affect its physical association with GIRK4, the routing of the heteromer to the cell surface, or heteromeric channel function, unlike the dramatic functional effects of N-glycosylation of ROMK1 at Asn(117) (Schwalbe, R. A., Wang, Z., Wible, B. A., and Brown, A. M. (1995) J. Biol. Chem. 270, 15336-15340).

Isolation and separation of proteoglycans

Proteoglycans contain a polypeptide core and an oligosaccharide chain composed of aminohexoses and uronic acid. The glycan chain is attached to the polypeptide in a bond to serine hydroxyl. The glycan chains may contain up to 200 disaccharide units and the proteoglycan molecular mass ranges from a few thousands to millions. Their physiological functions are related to barriers limiting diffusion across the membranes, articular lubrification, blood coagulation and cellular adhesion. The tissue proteoglycans can be extracted with 4 M guanidine hydrochloride and purified with chromatographic techniques. The soluble proteoglycans can be precipitated with cetylpyridinium chloride, purified by chromatography or by dialysis. All proteoglycan species are amenable to electrophoresis on polyacrylamide gels, and after blotting on polyvinylidene fluoride membranes, they can be stained for glycans. Proteoglycan analyses have shown their value in clinical mucopolysaccharidosis diagnostics, in occupational toxicology and in coagulation studies. Experimental applications include cell adhesion studies in tumor biology, regeneration in neurosciences or maturation of skin and kidneys.

The channel opening rate of adult- and fetal-type mouse muscle nicotinic receptors activated by acetylcholine

1. In this paper, we examine acetylcholine (ACh)-induced currents in quail fibroblast cell lines expressing either the fetal (Q-F18) or the adult (Q-A33) complement of nicotinic acetylcholine receptor subunits derived from mouse skeletal muscle. Pulses of ACh were applied to outside-out patches of cell membrane by means of a fast perfusion system at concentrations from 100 nM to 10 mM. We obtained current records with intracellular potentials of -60 and +40 mV. The goal of this study was to estimate the channel opening rate. 2. By fitting sums of exponentials to averaged responses, we estimated the rate of development of the current on the application of acetylcholine. The rate constant of the predominant exponential component (the on-rate) ranges over 3 orders of magnitude, from around 100 s-1 (fetal) at low concentrations ACh to over 100,000 s-1 (fetal and adult) at the highest concentrations. 3. We establish that our measurement of the on-rate is not limited by technical constraints, and can therefore be related to the rate constants of a kinetic scheme. Our observations are consistent with a model having a rate-limiting channel opening step with a forward rate constant (beta) of 80,000 s-1 on average for adult receptors and 60,000 s-1 for fetal receptors, and a minimum opening to closing ratio (beta/alpha) of around 33 (adult) or 50 (fetal). The channel opening rate, beta, varies from around 30,000 s-1 to well over 100,000 s-1 for different patches. The large variation cannot all be ascribed to errors of measurement, but indicates patch to patch variation.

Two types of ACh receptors contribute to fast channel gating on mouse skeletal muscle

Single-channel recordings from mouse C2 myotubes indicate that maturation of skeletal muscle is accompanied by the appearance of two types of fast acetylcholine (ACh) receptor channels that are each functionally distinct from the embryonic receptor type present at early stages of differentiation. The embryonic receptor type has a low conductance (45 pS) and long channel open time, rendering slowly decaying synaptic currents. One fast channel type that appears during muscle maturation is distinguished from the embryonic receptor type on the basis of both higher conductance (65 pS) and shorter open time. However, single-channel recordings from differentiated mouse skeletal muscle cell line (C2) point to the existence of a second fast receptor type, which has a conductance similar to the embryonic receptor type (45 pS), yet significantly reduced mean channel open time. Analyses of individual channel function at high ACh concentrations directly demonstrate the coexistence of two kinetically distinct types of 45 pS ACh receptors. Openings by fast type and slow embryonic type of 45 pS receptors occurred in bursts, allowing distinction on the basis of both mean open time and open probability for individual receptors. The embryonic type of 45 pS receptor has an open time approximately twofold longer than the fast-receptor counterpart. Additional differences were reflected in the open probability distributions for fast and slow 45 pS receptor types. Both types of 45 pS receptor were kinetically distinguishable from the 65 pS receptor. We found no support for the idea that the slow and fast 45 pS receptor types result from the interconversion of dual gating modes involving the same receptor protein. Our results are consistent with the idea that the acquisition of fast synaptic current decay, required at mature neuromuscular synapses, is the result of the up-regulation of two distinct fast types of nicotinic ACh receptors during skeletal muscle development.

Pharmacological and physiological properties of a putative ganglionic nicotinic receptor, alpha 3 beta 4, expressed in transfected eucaryotic cells

Neuronal nicotinic acetylcholine receptor subunits alpha 3 (PCA48E) and beta 4S (ZPC13) were expressed in human embryonic kidney (HEK)-293 cells by calcium phosphate transfection. In the presence of atropine, acetylcholine (ACh) induced fast activating currents which exhibited desensitization and inward rectification. The EC50 for ACh was 202 +/- 32 microM with a Hill coefficient of 1.9 +/- 0.4. The rank order of nicotinic agonist potency was 1,1-dimethyl-4-phenylpiperozinium (DMPP) > cytisine = nicotine approximately equal to ACh. The maximal response elicited by DMPP was substantially less than that elicited by other agonists, suggesting that DMPP is a partial agonist. ACh (500 microM) responses were very effectively blocked by equimolar concentrations (100 microM) of the ganglionic antagonists d-tubocurarine, mecamylamine and hexamethonium. Equal concentrations of the potent muscle receptor antagonist decamethonium and the competitive antagonist dihydro-beta-erythroidine were much less effective. alpha bungaro-toxin (1 microM) had little effect on ACh-induced responses. This physiological and pharmacological profile is consistent with a ganglionic nicotinic response.

Detailed structural analysis of asparagine-linked oligosaccharides of the nicotinic acetylcholine receptor from Torpedo californica

The structures of the major oligosaccharide moieties of the nicotinic acetylcholine receptor (AcChoR) protein from Torpedo californica have been reported [Nomoto, H., Takahashi, N., Nagaki, Y., Endo, S., Arata, Y. and Hayashi, K. (1986) Eur. J. Biochem. 157, 233-242] to be high-mannose types. Here we report detailed analyses of the structures of the remaining oligosaccharides in this receptor. The sialylated oligosaccharides released by glycopeptidase (almond) digestion were separated according to the number of sialic acid residues using high-performance anion-exchange chromatography with pulsed amperometric detection. After removal of sialic acid from each fraction, the resulting neutral oligosaccharides were separately pyridylaminated and were analyzed by a combination of sequential exoglycosidase digestion and HPLC, then identified on a two-dimensional sugar map. The structures of two desialylated pyridylamino-oligosaccharides were further analyzed by high-resolution proton NMR. Each oligosaccharide was composed of species containing varying numbers of sialic acids. The desialylated complex-type oligosaccharides of AcChoR consisted of ten, eight and one different biantennary, triantennary and tetraantennary oligosaccharide, respectively. The biantennary oligosaccharides were divided into two groups; oligosaccharides with fucose at the proximal N-acetylglucosamine (six varieties) and oligosaccharides without fucose (four varieties). Each group consisted of species differing in the number of terminal galactose residues. The major component of the biantennary oligosaccharides had two galactose residues at the non-reducing termini. The terminal alpha-galactose residue(s) linked to C3 of beta-galactose were found in the fucose-containing biantennary oligosaccharides (two varieties). The triantennary oligosaccharides were also divided into two groups; oligosaccharides with (four varieties) and without (four varieties) besecting N-acetylglucosamine. These groups were composed of species differing in the number of terminal galactose residues. The major component of the triantennary oligosaccharides was fully galactosylated with three galactose residues. An unusual group, Gal beta 1-3GlcNAc, was present in low levels in the triantennary oligosaccharides. In contrast, the tetraantennary oligosaccharide was composed of only one species, which is fully galactosylated with four galactose residues.

Effect of inhibitors of N-linked oligosaccharide processing on the high-affinity transport of D-aspartate by C6 glioma cells

The effect of several inhibitors of oligosaccharide-processing on the high-affinity transport of D-aspartate was investigated in C6 glioma cells. Swainsonine, an inhibitor of mannosidase II, had no effect on the uptake of the amino acid. Castanospermine (100 micrograms/ml) and 1-deoxynojirimycin (1 mM), inhibitors of glucosidases, and 1-deoxymannojirimycin (1 mM), an inhibitor of mannosidase I, reduced the rate of transport by 35-45%. All inhibitory compounds decreased the Vmax for transport without affecting the Km which suggests that inhibition of oligosaccharide trimming reduces the number of competent transporters on the surface of the plasma membrane. Returning the cells to a drug-free medium for 24 h, following a 24 h exposure, resulted in complete recovery of uptake. Treatment of cells with neuraminidase from V. cholerae also decreased the Vmax for transport by about 20%. The results suggest that: (i) a partial complex carbohydrate chain on the high-affinity transporter for acidic amino acid transmitters is sufficient for activity and (ii) sialic acid residues may be necessary for normal operation of the transporter.

Activation of skeletal muscle nicotinic acetylcholine receptors

Work over the past ten years has greatly increased our understanding of both the structure and function of the muscle nicotinic acetylcholine receptor. There is a strongly supported general picture of how the receptor functions: agonist binds rapidly to sites of low affinity and channel opening occurs at a rate comparable to the agonist dissociation rate. Channel closing is slow, so the channel has a high probability of being open if both agonist-binding sites are occupied by ACh. Results of expression studies have shown that each subunit can influence AChR activation and have given a structural basis for the major physiological change known for muscle AChR, the developmental change in AChR activation. These general statements notwithstanding, there are still major areas of uncertainty which limit our understanding. We have emphasized these areas of uncertainty in this review, to indicate what needs to be done. First, the quantitative estimates of rate constants are not as strongly supported as they should be. The major reasons are twofold--uncertainties about the interpretation of components in the kinetic data and difficulties of resolving brief events. As a result, any inferences about the functional consequences of structural alterations must remain tenuous. Second, the functional behavior of individual AChRs is not as well understood as it should be. The kinetic behavior of an individual receptor clearly can be complex (section II). In addition, there is evidence that superimposed on this complexity there may be stable and kinetically distinguishable populations of receptors (section III). Until the basis for the kinetically defined populations is clarified, kinetic parameters for receptors of defined structure cannot be unambiguously obtained. Finally, it is not surprising that the studies of AChR of altered structure have not given definitive results. Two reasons should be apparent from the preceding points: there is not a fully supported approach for kinetic analysis, and the "normal" population may not be clearly defined. An additional complication is also emerging, in that the available data support the idea that specific residues distributed over all subunits may influence AChR activation. This possibility renders the task of analysis that much more difficult. The muscle nicotinic AChR has served as a prototype for the family of transmitter-gated membrane channels, which includes the muscle and neuronal nicotinic receptors, the GABAA, the glycine and possibly the non-NMDA excitatory amino acid receptor (Stroud et al., 1990). It is interesting to note that the functional properties of the GABAA receptor, probably the best-studied of the other members of the family are rather similar.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional properties of acetylcholine receptors coexpressed with the 43K protein in heterologous cell systems

The nicotinic acetylcholine (ACh) receptor is an integral membrane protein which mediates synaptic transmission at the skeletal neuromuscular junction. A key event in the development of the neuromuscular junction is the formation of high density aggregates of ACh receptors in the postsynaptic membrane. Receptor clustering has been attributed, in part, to their association with a peripheral membrane protein of Mr 43,000 (43K protein). We have addressed whether the association of the 43K protein can alter the single channel properties of the ACh receptor, and thus influence neuromuscular transmission at developing synapses, by expressing ACh receptors with and without the 43K protein in heterologous expression systems. We found that coexpression of the 43K protein with the receptor did not significantly alter either its single channel conductance or its mean channel open time. This was true in oocytes and also in COS cells where it was possible to localize 43K-induced clusters by fluorescence microscopy and to record from those clustered receptors. These data are in agreement with previous single channel studies which have shown that the properties of diffusely distributed and clustered receptors in native muscle cells from both mice and Xenopus do not differ.

ACh receptor-rich membrane domains organized in fibroblasts by recombinant 43-kildalton protein

Neurotransmitter receptors are generally clustered in the postsynaptic membrane. The mechanism of clustering was analyzed with fibroblast cell lines that were stably transfected with the four subunits for fetal (alpha, beta, gamma, delta) or adult (alpha, beta, epsilon, delta) type mouse muscle nicotinic acetylcholine receptors (AChRs). Immunofluorescent staining indicated that AChRs were dispersed on the surface of these cells. When transiently transfected with an expression construct encoding a 43-kilodalton protein that is normally concentrated under the postsynaptic membrane, AChRs expressed in these cells became aggregated in large cell-surface clusters, colocalized with the 43-kilodalton protein. This suggests that 43-kilodalton protein can induce AChR clustering and that cluster induction involves direct contact between AChR and 43-kilodalton protein.

Assembly intermediates of the mouse muscle nicotinic acetylcholine receptor in stably transfected fibroblasts

We have used fibroblast clones expressing muscle nicotinic acetylcholine receptor alpha and gamma, and alpha and delta subunits to measure the kinetics of subunit assembly, and to study the properties of the partially assembled products that are formed. We demonstrate by coimmunoprecipitation that assembly intermediates in fibroblasts coexpressing alpha and delta subunits are formed in a time-dependent manner. The alpha and gamma- and the alpha and delta-producing transfected cells form complexes that, when labeled with 125I-alpha-bungarotoxin, migrate in sucrose gradients at 6.3S, a value consistent with a hetero-dimer structure. An additional peak at 8.5S is formed from the alpha and gamma subunits expressed in fibroblasts suggesting that gamma may have more than one binding site for alpha subunit. The stability and specificity of formation of these partially assembled complexes suggests that they are normal intermediates in the assembly of acetylcholine receptor. Comparison of the binding of 125I-alpha-bungarotoxin to intact and detergent-extracted fibroblasts indicate that essentially all of the binding sites are retained in an intracellular pool. The fibroblast delta subunit has the electrophoretic mobility in SDS-PAGE of a precursor that does not contain complex carbohydrates. In addition, alpha gamma and alpha delta complexes had lectin binding properties expected of subunits lacking complex oligosaccharides. Therefore, fibroblasts coexpressing alpha and gamma or alpha and delta subunits produce discrete assembly intermediates that are retained in an intracellular compartment and are not processed by Golgi enzymes.

Influence of the gamma subunit and expression system on acetylcholine receptor gating

We have developed a partial kinetic theory for the gating of murine nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes and have used this theory to characterize the role of the gamma subunit in single-channel behavior. Permeation and gating were found to be largely unaffected in AChRs produced in oocytes when the gamma subunit transcript was omitted from microinjections of AChR subunit RNAs. In contrast, marked changes in gating kinetics resulted when even very conservative single amino acid substitutions were introduced into the gamma subunit, indicating that the gamma subunit can have a large effect on AChR gating. We also found that channel openings were much prolonged when murine AChRs were expressed in BC3H-1 cells.

Excision of membrane patches reduces the mean open time of nicotinic acetylcholine receptors

Excision of membrane patches from clonal BC3H1 cells to the inside-out configuration reduces the mean open time and burst duration of skeletal muscle nicotinic acetylcholine receptors. The channel conductance is not altered. Our results indicate that the rates for several steps in receptor activation are affected. The altered receptors apparently form a homogeneous population. Patterns of activity similar to that seen after excision are seen infrequently in recordings from cell-attached patches, suggesting that receptors with this type of altered function are present in intact membranes. The extent and rate of the reduction in mean open time after excision are similar at acetylcholine concentration over a 1000-fold range, indicating that the alteration does not depend on the state of ligation of the receptor. The extent and rate also are similar at room temperature and at 11 degrees C. The changes seen indicate that careful comparisons to intact preparations are necessary when studying the gating of channels in excised patches or following reconstitution into artificial membranes.

Expression of Torpedo nicotinic acetylcholine receptor subunits in yeast is enhanced by use of yeast signal sequences

We have produced the four subunits of the nicotinic acetylcholine receptor of Torpedo californica, an integral membrane protein, in the yeast Saccharomyces cerevisiae. Two of the subunits (alpha and delta) were readily produced from their cDNAs after simply subcloning them into a yeast shuttle vector adjacent to a yeast promoter. The other two protein subunits (beta and gamma) were not produced by this strategy, although the amounts of mRNA produced from these expression constructs are similar to those for alpha and delta. Replacing the DNA coding for the normal N-terminal signal sequences for the beta and gamma subunits with DNA coding for the signal sequence of yeast invertase results in successful protein synthesis. The yeast signal sequence allows these subunits to be translocated across the membrane of the endoplasmic reticulum and to be glycosylated. The appropriate final size of the subunit proteins suggests that the yeast signal sequence has been properly cleaved after translocation.

Separation and partial characterization of neuropeptide-inducing factors in heart cell conditioned medium

Various conditioned media contain multiple factors that regulate the expression of the neurotransmitters acetylcholine, serotonin, and catecholamines and the neuropeptides substance P, somatostatin, vasoactive intestinal polypeptide-related peptides, cholecystokinin, and enkephalins in cultured sympathetic neurons. Using biochemical and immunological methods, we identify at least three distinct factors in heart cell conditioned medium: one induces acetylcholine, substance P, somatostatin, and vasoactive intestinal polypeptide-related peptides while suppressing catecholamine expression, a second factor induces only vasoactive intestinal polypeptide-related peptides, and a third factor induces only somatostatin expression. These observations demonstrate the existence of a group of biochemically and immunologically distinct factors involved in phenotypic specification with unique, but partially overlapping activities. The analogy with the family of differentiation factors in the hematopoietic system is discussed.