Membrane dynamics and the biogenesis of lysosomes (Review)

Lysosomes are dynamic organelles receiving membrane traffic input from the biosynthetic, endocytic and autophagic pathways. They may be regarded as storage organelles for acid hydrolases and are capable of fusing with late endosomes to form hybrid organelles where digestion of endocytosed macromolecules occurs. Reformation of lysosomes from the hybrid organelles involves content condensation and probably removal of some membrane proteins by vesicular traffic. Lysosomes can also fuse with the plasma membrane in response to cell surface damage and a rise in cytosolic Ca 2+ concentration. This process is important in plasma membrane repair. The molecular basis of membrane traffic pathways involving lysosomes is increasingly understood, in large part because of the identification of many proteins required for protein traffic to vacuoles in the yeast Saccharomyces cerevisiae. Mammalian orthologues of these proteins have been identified and studied in the processes of vesicular delivery of newly synthesized lysosomal proteins from the trans-Golgi network, fusion of lysosomes with late endosomes and sorting of membrane proteins into lumenal vesicles. Several multi-protein oligomeric complexes required for these processes have been identified. The present review focuses on current understanding of the molecular mechanisms of fusion of lysosomes with both endosomes and the plasma membrane and on the sorting events required for delivery of newly synthesized membrane proteins, endocytosed membrane proteins and other endocytosed macromolecules to lysosomes.     

Membrane dynamics and the biogenesis of lysosomes

Lysosomes are dynamic organelles receiving membrane traffic input from the biosynthetic, endocytic and autophagic pathways. They may be regarded as storage organelles for acid hydrolases and are capable of fusing with late endosomes to form hybrid organelles where digestion of endocytosed macromolecules occurs. Reformation of lysosomes from the hybrid organelles involves content condensation and probably removal of some membrane proteins by vesicular traffic. Lysosomes can also fuse with the plasma membrane in response to cell surface damage and a rise in cytosolic Ca(2+) concentration. This process is important in plasma membrane repair. The molecular basis of membrane traffic pathways involving lysosomes is increasingly understood, in large part because of the identification of many proteins required for protein traffic to vacuoles in the yeast Saccharomyces cerevisiae. Mammalian orthologues of these proteins have been identified and studied in the processes of vesicular delivery of newly synthesized lysosomal proteins from the trans-Golgi network, fusion of lysosomes with late endosomes and sorting of membrane proteins into lumenal vesicles. Several multi-protein oligomeric complexes required for these processes have been identified. The present review focuses on current understanding of the molecular mechanisms of fusion of lysosomes with both endosomes and the plasma membrane and on the sorting events required for delivery of newly synthesized membrane proteins, endocytosed membrane proteins and other endocytosed macromolecules to lysosomes.     

Assembly-dependent trafficking assays in the detection of receptor-receptor interactions

Assembly-dependent trafficking is a property of many multimeric membrane protein complexes; this coupling of assembly and trafficking processes provides an important cellular quality control mechanism, ensuring that only properly folded and assembled complexes are expressed on the cell surface. In all membrane protein complexes whose trafficking is known to be assembly-dependent, at least one of the subunits contains an endoplasmic reticulum (ER) retention/retrieval signal that is shielded on subunit assembly, allowing the assembled protein complex to traffic to the plasma membrane. Under these conditions, presence of the normally retained subunit on the cell surface can be used as an indirect index of protein assembly in the ER. In this article, I describe the design of two complementary approaches (trafficking enhancement and trap assays) that can be used separately or in combination to determine whether two (or more) proteins assemble in the ER, i.e., whether they constitutively oligomerize. Both of the approaches are based on the measurement of plasma membrane-expressed proteins using antibody-mediated detection of extracellularly expressed epitopes and subsequent luminometric quantification. These methods provide a straightforward and relatively inexpensive way to assess protein-protein interactions early in the synthetic pathway.     

Cell surface targeting of VPAC1 receptors: evidence for implication of a quality control system and the proteasome

Like for most transmembrane proteins, translation of G protein-coupled receptors (GPCRs) mRNA takes place at the endoplasmic reticulum (ER) where they are synthesized, folded and assembled. The molecular mechanisms involved in the transport process of GPCRs from ER to the plasma membrane are poorly investigated. Here we studied the mechanisms involved in glycosylation-dependent cell surface expression and quality control of the receptor for Vasoactive Intestinal Polypeptide (VIP) VPAC1, a member of the B family of GPCRs. Using biochemical and pharmacological techniques and fluorescence microscopy, we have shown that only a fraction of newly synthesized VPAC1 attains properly conformation that allows their cell surface targeting. Misfolded or immature VPAC1 are taken in charge by co- and post-translational quality control that involves: 1) calnexin-dependent folding strictly through a glycan-dependent mechanism, 2) BiP-dependant folding, 3) translocation to the cytoplasm and proteasome-dependent degradation of improper proteins, and 4) post-ER quality control check points. Our data suggest that VPAC1 expression/trafficking pathways are under the control of complex and precise molecular mechanisms to ensure that only proper VPAC1 reaches the cell surface.     

Geranylgeranylated SNAREs are dominant inhibitors of membrane fusion

Exocytosis in yeast requires the assembly of the secretory vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (v-SNARE) Sncp and the plasma membrane t-SNAREs Ssop and Sec9p into a SNARE complex. High-level expression of mutant Snc1 or Sso2 proteins that have a COOH-terminal geranylgeranylation signal instead of a transmembrane domain inhibits exocytosis at a stage after vesicle docking. The mutant SNARE proteins are membrane associated, correctly targeted, assemble into SNARE complexes, and do not interfere with the incorporation of wild-type SNARE proteins into complexes. Mutant SNARE complexes recruit GFP-Sec1p to sites of exocytosis and can be disassembled by the Sec18p ATPase. Heterotrimeric SNARE complexes assembled from both wild-type and mutant SNAREs are present in heterogeneous higher-order complexes containing Sec1p that sediment at greater than 20S. Based on a structural analogy between geranylgeranylated SNAREs and the GPI-HA mutant influenza virus fusion protein, we propose that the mutant SNAREs are fusion proteins unable to catalyze fusion of the distal leaflets of the secretory vesicle and plasma membrane. In support of this model, the inverted cone-shaped lipid lysophosphatidylcholine rescues secretion from SNARE mutant cells.     

Subunit composition determines Kv1 potassium channel surface expression

Shaker-related or Kv1 voltage-gated K(+) channels play critical roles in regulating the excitability of mammalian neurons. Native Kv1 channel complexes are octamers of four integral membrane alpha subunits and four cytoplasmic beta subunits, such that a tremendous diversity of channel complexes can be assembled from the array of alpha and beta subunits expressed in the brain. However, biochemical and immunohistochemical studies have demonstrated that only certain complexes predominate in the mammalian brain, suggesting that regulatory mechanisms exist that ensure plasma membrane targeting of only physiologically appropriate channel complexes. Here we show that Kv1 channels assembled as homo- or heterotetrameric complexes had distinct surface expression characteristics in both transfected mammalian cells and hippocampal neurons. Homotetrameric Kv1.1 channels were localized to endoplasmic reticulum, Kv1.4 channels to the cell surface, and Kv1.2 channels to both endoplasmic reticulum and the cell surface. Heteromeric assembly with Kv1.4 resulted in dose-dependent increases in cell surface expression of coassembled Kv1.1 and Kv1.2, while coassembly with Kv1.1 had a dominant-negative effect on Kv1.2 and Kv1.4 surface expression. Coassembly with Kv beta subunits promoted cell surface expression of each Kv1 heteromeric complex. These data suggest that subunit composition and stoichiometry determine surface expression characteristics of Kv1 channels in excitable cells.     

Organelle biogenesis and intracellular lipid transport in eukaryotes.

The inter- and intramembrane transport of phospholipids, sphingolipids, and sterols involves the most fundamental processes of membrane biogenesis. Identification of the mechanisms involved in these lipid transport reactions has lagged significantly behind that for intermembrane protein traffic until recently. Application of methods that include fluorescently labeled and spin-labeled lipid analogs, new cellular fractionation techniques, topographically specific chemical modification techniques, the identification of organelle-specific metabolism, permeabilized cell methodology, and yeast molecular genetics has contributed to revealing a diverse biochemical array of transport processes for lipids. Compelling evidence now exists for ATP-dependent, ATP-independent, vesicle-dependent, and vesicle-independent transport processes that are lipid and membrane specific. ATP-dependent transport processes include the transbilayer movement of phosphatidylserine and phosphatidylethanolamine at the plasma membrane and the transport of phosphatidylserine from its site of synthesis to the mitochondria. ATP-independent processes include the transbilayer movement of virtually all lipids at the endoplasmic reticulum, the movement of phosphatidylserine between the inner and outer mitochondrial membranes, and the transfer of nascent phosphatidylcholine and phosphatidylethanolamine to the plasma membrane. The ATP-independent movement of lipids between organelles is believed to be due to the action of lipid transfer proteins, but this still remains to be proved. Vesicle-based transport mechanisms (which are also inherently ATP dependent) include the transport of nascent cholesterol, sphingomyelin, and glycosphingolipids from the Golgi apparatus to the plasma membrane and the recycling of sphingolipids and selected pools of phosphatidylcholine from the plasma membrane to the cell interior. The vesicles involved in cholesterol transport to the plasma membrane are different from those involved in bulk protein transport to the cell surface. The vesicles involved in recycling sphingomyelin to and from the cell surface are different from those involved in the assembly of newly synthesized sphingolipids into the plasma membrane. The preliminary characterization of these lipid translocation processes suggests divergent rather than unifying mechanisms for lipid transport in organelle assembly.     

Lipid fluidity-dependent biosynthesis and assembly of the outer membrane proteins of E. coli.

The reduction of the membrane lipids of E. coli to a nonfluid state resulted in the accumulation in the cell envelope of a high molecular weight precursor of the protoIG protein, a major outer membrane protein. The protoIG protein was as sensitive to trypsin as the mature toIG protein assembled in the outer membrane. In contrast to the toIG protein, however, the accumulated protoIG protein was easily released from the envelope fraction by both sodium lauryl sarcosinate extraction and sonication. This indicated that the precursor protein was loosely associated with the cell membrane. When a fluid lipid state was restored, the protoIG protein was processed to the mature form which was then correctly assembled in the outer membrane. These results suggest that the protoIG protein produced under nonfluid lipid conditions was properly translocated across the cytoplasmic membrane, but could not be assembled in the outer membrane due either to the reversible inhibition of the processing of the ProtoIG to the toIG protein or to the lack of interaction with a specific outer membrane component(s). Reduced lipid fluidity also caused various alterations in the biosynthesis and assembly of other membrane proteins. In addition to the toIG protein, a large number of new proteins were accumulated in the membrane. Alternatively, the matrix protein as well as the promatrix protein were not detected in the cell envelope. On the other hand, the lipoprotein was normally produced, processed, modified and assembled in the outer membrane. These results indicate that the outer membrane proteins are synthesized and assembled according to several different mechanisms, on which the physical state of the membrane has various effects.     

Nocodazole, a microtubule-active drug, interferes with apical protein delivery in cultured intestinal epithelial cells (Caco-2)

The polarized delivery of membrane proteins to the cell surface and the initial secretion of lysosomal proteins into the culture medium were studied in the polarized human intestinal adenocarcinoma cell line Caco-2 in the presence or absence of the microtubule-active drug nocodazole. The appearance of newly synthesized proteins at the plasma membrane was measured by their sensitivity to proteases added either to the apical or the basolateral surface of cells grown on nitrocellulose filters. Nocodazole was found to reduce the delivery to the cell surface of an apical membrane protein, aminopeptidase N, and to lead to its partial missorting to the basolateral surface, whereas the drug had no influence on the delivery of a basolateral 120-kD membrane protein defined by a monoclonal antibody. Furthermore, nocodazole selectively blocked the apical secretion of two lysosomal proteins, cathepsin D and acid alpha-glucosidase, whereas the drug had no influence on their basolateral secretion. These results suggest that in Caco-2 cells an intact microtubular network is important for the transport of newly synthesized proteins to the apical cell surface.     

Kinetics of intracellular transport and sorting of lysosomal membrane and plasma membrane proteins

We have used monospecific antisera to two lysosomal membrane glycoproteins, lgp120 and a similar protein, lgp110, to compare the biosynthesis and intracellular transport of lysosomal membrane components, plasma membrane proteins, and lysosomal enzymes. In J774 cells and NRK cells, newly synthesized lysosomal membrane and plasma membrane proteins (the IgG1/IgG2b Fc receptor or influenza virus hemagglutinin) were transported through the Golgi apparatus (defined by acquisition of resistance to endo-beta-N-acetylglucosaminidase H) with the same kinetics (t1/2 = 11-14 min). In addition, immunoelectron microscopy of normal rat kidney cells showed that lgp120 and vesicular stomatitis virus G-protein were present in the same Golgi cisternae demonstrating that lysosomal and plasma membrane proteins were not sorted either before or during transport through the Golgi apparatus. To define the site at which sorting occurred, we compared the kinetics of transport of lysosomal and plasma membrane proteins and a lysosomal enzyme to their respective destinations. Newly synthesized proteins were detected in dense lysosomes (lgp's and beta-glucuronidase) or on the cell surface (Fc receptor or hemagglutinin) after the same lag period (20-25 min), and accumulated at their final destinations with similar kinetics (t1/2 = 30-45 min), suggesting that these two lgp's are not transported to the plasma membrane before reaching lysosomes. This was further supported by measurements of the transport of membrane-bound endocytic markers from the cell surface to lysosomes, which exhibited additional lag periods of 5-15 min and half-times of 1.5-2 h. The time required for transport of newly synthesized plasma membrane proteins to the cell surface, and for the transport of plasma membrane markers from the cell surface to lysosomes would appear too long to account for the rapid transport of lgp's from the Golgi apparatus to lysosomes. Thus, the observed kinetics suggest that lysosomal membrane proteins are sorted from plasma membrane proteins at a post-Golgi intracellular site, possibly the trans Golgi network, before their delivery to lysosomes.     

Entry of newly synthesized GLUT4 into the insulin-responsive storage compartment is GGA dependent

Following biosynthesis, both GLUT1 and VSV-G proteins appear rapidly (2-3 h) at the plasma membrane, whereas GLUT4 is retained in intracellular membrane compartments and does not display any significant insulin responsiveness until 6-9 h. Surprisingly, the acquisition of insulin responsiveness did not require plasma membrane endocytosis, as expression of a dominant-interfering dynamin mutant (Dyn/K44A) had no effect on the insulin-stimulated GLUT4 translocation. Furthermore, expression of endocytosis-defective GLUT4 mutants or continuous surface labeling with an exofacial specific antibody demonstrated that GLUT4 did not transit the cell surface prior to the acquisition of insulin responsiveness. The expression of a dominant-interfering GGA mutant (VHS-GAT) had no effect on the trafficking of newly synthesized GLUT1 or VSV-G protein to the plasma membrane, but completely blocked the insulin-stimulated translocation of newly synthesized GLUT4. Furthermore, in vitro budding of GLUT4 vesicles but not GLUT1 or the transferrin receptor was inhibited by VHS-GAT. Together, these data demonstrate that following biosynthesis, GLUT4 directly sorts and traffics to the insulin-responsive storage compartment through a specific GGA-sensitive process.     

Vpu and Tsg101 regulate intracellular targeting of the human immunodeficiency virus type 1 core protein precursor Pr55gag

Assembly of human immunodeficiency virus type 1 (HIV-1) is directed by the viral core protein Pr55gag. Depending on the cell type, Pr55gag accumulates either at the plasma membrane or on late endosomes/multivesicular bodies. Intracellular localization of Pr55gag determines the site of virus assembly, but molecular mechanisms that define cell surface or endosomal targeting of Pr55gag are poorly characterized. We have analyzed targeting of newly synthesized Pr55gag in HeLa H1 cells by pulse-chase studies and subcellular fractionations. Our results indicated that Pr55gag was inserted into the plasma membrane and, when coexpressed with the viral accessory protein Vpu, Pr55gag remained at the plasma membrane and virions assembled at this site. In contrast, Pr55gag expressed in the absence of Vpu was initially inserted into the plasma membrane, but subsequently endocytosed, and virus assembly was partially shifted to internal membranes. This endocytosis of Pr55gag required the host protein Tsg101. These results identified a previously unknown role for Vpu and Tsg101 as regulators for the endocytic uptake of Pr55gag and suggested that the site of HIV-1 assembly is determined by factors that regulate the endocytosis of Pr55gag.     

Sperm membrane physiology and relevance for fertilization

This paper aims to overview recent insights in sperm surface remodelling pertinent to fertilization. A basic understanding of this remodelling is required to interpret the high amount of data appearing from high-throughput identification techniques for proteins presently applied in reproductive biology. From the extensive lists of protein candidates identified by proteomics, only a few are recognized to be directly involved in fertilization. Others are indirectly involved, but many are not yet considered to be involved in fertilization. Some of these newly identified and unexpected proteins may shed new light in the current molecular models for fertilization. However, the gathered lists of sperm proteins possibly involved in fertilization do only tell a part of the story regarding how fertilization is accomplished. When considering the identification of proteins involved in fertilization, one also needs to take into account the fundamental mechanisms involved in the redistribution of sperm surface proteins in membrane protein complexes and the involvement of cell signalling events that regulate their post-translational modification status. Both processes are likely requisite for protein configuration and grouping into functional membrane protein complexes necessary to elicit their delicate roles in fertilization. This paper emphasizes biochemical models for membrane surface modelling and their potential involvement for remodelling the sperm surface in the above described processes.     

Envelope glycoprotein interactions in coronavirus assembly

Coronaviruses are assembled by budding into smooth membranes of the intermediate ER-to-Golgi compartment. We have studied the association of the viral membrane glycoproteins M and S in the formation of the virion envelope. Using coimmunoprecipitation analysis we demonstrated that the M and S proteins of mouse hepatitis virus (MHV) interact specifically forming heteromultimeric complexes in infected cells. These could be detected only when the detergents used for their solubilization from cells or virions were carefully chosen: a combination of nonionic (NP-40) and ionic (deoxycholic acid) detergents proved to be optimal. Pulse-chase experiments revealed that newly made M and S proteins engaged in complex formation with different kinetics. Whereas the M protein appeared in complexes immediately after its synthesis, newly synthesized S protein did so only after a lag phase of > 20 min. Newly made M was incorporated into virus particles faster than S, which suggests that it associates with preexisting S molecules. Using the vaccinia virus T7-driven coexpression of M and S we also demonstrate formation of M/S complexes in the absence of other coronaviral proteins. Pulse-chase labelings and coimmunoprecipitation analyses revealed that M and S associate in pre-Golgi membranes because the unglycosylated form of M appeared in M/S complexes rapidly. Since no association of M and S was detected when protein export from the ER was blocked by brefeldin A, stable complexes most likely arise in the ER-to-Golgi intermediate compartment. Sucrose velocity gradient analysis showed the M/S complexes to be heterogeneous and of higher order, suggesting that they are maintained by homo- and heterotypic interactions. M/S complexes colocalized with alpha-mannosidase II, a resident Golgi protein. They acquired Golgi-specific oligosaccharide modifications but were not detected at the cell surface. Thus, the S protein, which on itself was transported to the plasma membrane, was retained in the Golgi complex by its association with the M protein. Because coronaviruses bud at pre-Golgi membranes, this result implies that the envelope glycoprotein complexes do not determine the site of budding. Yet, the self-association of the MHV envelope glycoproteins into higher order complexes is indicative of its role in the sorting of the viral membrane proteins and in driving the formation of the viral lipoprotein coat in virus assembly.     

Synthesis and assembly of membrane skeletal proteins in mammalian red cell precursors

The synthesis of membrane skeletal proteins in avian nucleated red cells has been the subject of extensive investigation, whereas little is known about skeletal protein synthesis in bone marrow erythroblasts and peripheral blood reticulocytes in mammals. To address this question, we have isolated nucleated red cell precursors and reticulocytes from spleens and from the peripheral blood, respectively, of rats with phenylhydrazine-induced hemolytic anemia and pulse-labeled them with [35S]methionine. Pulse-labeling of nucleated red cell precursors shows that the newly synthesized alpha- and beta-spectrins are present in the cytosol, with a severalfold excess of alpha-spectrin over beta-spectrin. However, in the membrane-skeletal fraction, newly synthesized alpha- and beta-spectrins are assembled in stoichiometric amounts, suggesting that the association of alpha-spectrin with the membrane skeleton may be rate-limited by the amount of beta-spectrin synthesized, as has been shown recently in avian erythroid cells (Blikstad, I., W. J. Nelson, R. T. Moon, and E. Lazarides, 1983. Cell, 32:1081-1091). Pulse-chase experiments in the rat nucleated red cell precursors show that the newly synthesized alpha- and beta-spectrin of the cytosol turn over coordinately and extremely rapidly. In contrast, in the membrane-skeletal fraction, the newly synthesized polypeptides of spectrin are stable. In contrast to nucleated erythroid cells, in reticulocytes the synthesis of alpha- and beta-spectrins is markedly diminished compared with the synthesis and assembly of proteins comigrating with bands 2.1 and 4.1 on SDS gels. Thus, in nucleated red cell precursors, the newly synthesized spectrin may be attached to the plasma membrane before proteins 2.1 and 4.1 are completely synthesized and incorporated in the membrane.     

Photochemical competence of assembled photosystem II core complex in cyanobacterial plasma membrane

Cyanobacterial cells have two autonomous internal membrane systems, plasma membrane and thylakoid membrane. In these oxygenic photosynthetic organisms the assembly of the large membrane protein complex photosystem II (PSII) is an intricate process that requires the recruitment of numerous protein subunits and cofactors involved in excitation and electron transfer processes. Precise control of this assembly process is necessary because electron transfer reactions in partially assembled PSII can lead to oxidative damage and degradation of the protein complex. In this communication we demonstrate that the activation of PSII electron transfer reactions in the cyanobacterium Synechocystis sp. PCC 6803 takes place sequentially. In this organism partially assembled PSII complexes can be detected in the plasma membrane. We have determined that such PSII complexes can undergo light-induced charge separation and contain a functional electron acceptor side but not an assembled donor side. In contrast, PSII complexes in thylakoid membrane are fully assembled and capable of multiple turnovers. We conclude that PSII reaction center cores assembled in the plasma membrane are photochemically competent and can catalyze single turnovers. We propose that upon transfer of such PSII core complexes to the thylakoid membrane, additional proteins are incorporated followed by binding and activation of various donor side cofactors. Such a stepwise process protects cyanobacterial cells from potentially harmful consequences of performing water oxidation in a partially assembled PSII complex before it reaches its final destination in the thylakoid membrane.     

Surface architecture of the plant cell: Biogenesis of the cell wall,with special emphasis on the role of the plasma membrane in cellulose biosynthesis

Cell wall structure and biogenesis in the unicellular green alga, Oocystis apiculata, is described. The wall consists of an outer amourphous primary layer and an inner secondary layer of highly organized cellulosic microfibrils. The primary wall is deposited immediately after cytokinesis. Golgi-derived products contribute to this layer. Cortical microtubules underlie the plasma membrane immediately before and during primary wall formation. They function in maintaining the elliptical cell shape. Following primary wall synthesis, Golgi-derived materials accumulate on the cell surface to form the periplasmic layer. This layer functions in the deposition of coating and cross-linking substances which associate with cellulosic microfibrils of the incipient secondary wall. Secondary wall microfibrils are assembled in association with the plasma membrane. Freeze-etch preparations of untreated, living cells reveal linear terminal complexes in association with growing cellulosic microfibrils. These complexes are embedded in the EF fracture face of the plasma membrane. The newly synthesized microfibril lies in a groove of the outer leaflet of the plasma membrane. The groove is decorated on the EF fracture face by perpendicular structures termed “ridges.” The ridges interlink with definitive rows of particles associated with the PF fracture face of the inner leaflet of the plasma membrane. These particles are termed “granule bands,” and they function in the orientation of the newly synthesized microfibrils. Microfibril development in relation to a coordinated multienzyme complex is discussed. The process of cell wall biogenesis in Oocystis is compared to that in higher plants.     

A trafficking checkpoint controls GABA(B) receptor heterodimerization

Surface expression of GABA(B) receptors requires heterodimerization of GB1 and GB2 subunits, but little is known about mechanisms that ensure efficient heterodimer assembly. We found that expression of the GB1 subunit on the cell surface is prevented through a C-terminal retention motif RXR(R); this sequence is reminiscent of the ER retention/retrieval motif RKR identified in subunits of the ATP-sensitive K+ channel. Interaction of GB1 and GB2 through their C-terminal coiled-coil alpha helices masks the retention signal in GB1, allowing the plasma membrane expression of the assembled complexes. Because individual GABA(B) receptor subunits and improperly assembled receptor complexes are not functional even if expressed on the cell surface, we conclude that a trafficking checkpoint ensures efficient assembly of functional GABA(B) receptors.