Expression of hybrid (Na+ + K+)-ATPase molecules after transfection of mouse Ltk-cells with DNA encoding the beta-subunit of an avian brain sodium pump

Expression of the sarco/endoplasmic Ca2+-ATPase, SERCA1a, in fibroblasts induces the formation of organelle membrane arrays

Members of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) family are transmembrane proteins that are essential for the function of intracellular Ca(2+) storage organelles. We found that overexpression of avian muscle SERCA1a in transfected mouse fibroblasts led to the appearance of tubular membrane bundles that we termed plaques. These structures were generated in transfected cells when SERCA1a protein expression approached the endogenous level measured in chicken skeletal muscle. Plaque membranes had associated ribosomes and contained endoplasmic reticulum (ER) proteins. Endogenous ER protein levels were not elevated in SERCA1a-expressing cells, indicating that plaques were not generalized proliferations of ER but rather a reorganization of existing organelle membrane. Plaque formation also was observed in cells expressing a green fluorescent protein-SERCA1a fusion protein (GFP-SERCA1a). GFP-SERCA1a molecules displayed extensive lateral mobility between plaques, suggesting the presence of membrane continuities between these structures. Plaques were induced in cells expressing cDNA encoding a catalytically silent SERCA1a mutant indicating that ER redistribution was driven by a structural feature of the enzyme. SERCA1a-induced plaque formation shares some characteristics of sarcoplasmic reticulum (SR) biogenesis during muscle differentiation, and high-level SERCA1a expression in vivo may contribute to the formation of SR from ER during embryonic myogenesis.

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.

Apical plasma membrane mispolarization of NaK-ATPase in polycystic kidney disease epithelia is associated with aberrant expression of the beta2 isoform

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disease of the kidney, characterized by cystic enlargement of renal tubules, aberrant epithelial proliferation, and ion and fluid secretion into the lumen. Previous studies have shown abnormalities in polarization of membrane proteins, including mislocalization of the NaK-ATPase to the apical plasma membranes of cystic epithelia. Apically located NaK-ATPase has previously been shown to be fully functional in vivo and in membrane-grown ADPKD epithelial cells in vitro, where basal-to-apical (22)Na transport was inhibited by application of ouabain to the apical membrane compartment. Studies were conducted with polymerase chain reaction-generated specific riboprobes and polyclonal peptide antibodies against human sequences of alpha1, alpha3, beta1, and beta2 subunits of NaK-ATPase. High levels of expression of alpha1 and beta1 messenger RNA were detected in ADPKD and age-matched normal adult kidneys in vivo, whereas beta2 messenger RNA was detected only in ADPKD kidneys. Western blot analysis and immunocytochemical studies showed that, in normal adult kidneys, peptide subunit-specific antibodies against alpha1 and beta1 localized to the basolateral membranes of normal renal tubules, predominantly thick ascending limbs of Henle's loop. In ADPKD kidneys, alpha1 and beta2 subunits were localized to the apical epithelial cell membranes, whereas beta1 was distributed throughout the cytoplasm and predominantly in the endoplasmic reticulum, but was not seen associated with cystic epithelial cell membranes or in cell membrane fractions. Polarizing, renal-derived epithelial Madin Darby canine kidney cells, stably expressing normal or N-terminally truncated chicken beta1 subunits, showed selective accumulation in the basolateral Madin Darby canine kidney cell surface, whereas c-myc epitope-tagged chicken beta2 or human beta2 subunits accumulated selectively in the apical cell surface. Similarly, human ADPKD epithelial cell lines, which endogenously expressed alpha1 and beta2 NaK-ATPase subunits, showed colocalization at the apical cell surface and coassociation by immunoprecipitation analysis. These results are consistent with a model in which the additional transcription and translation of the beta2 subunit of NaK-ATPase may result in the apical mislocalization of NaK-ATPase in ADPKD cystic epithelia.

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

Insect Na(+)/K(+)-ATPase

Na(+)/K(+)-ATPase (sodium/potassium pump) is a P-type ion-motive ATPase found in the plasma membranes of animal cels. In vertebrates, the functions of this enzyme in nerves, heart and kidney are well characterized and characteristics a defined by different isoforms. In contrast, despite different tissue distributions, insects possess a single isoform of the alpha-subunit. A comparison of insect and vertebrate Na(+)/K(+)-ATPases reveals that although the mode of action and structure are very highly conserved, the specific roles of the enzyme in most tissues varies. However, the enzyme is essential for the function of nerve cells, and in this respect Na(+)/K(+)-ATPase appears to be fundamental in metazoan evolution.

The Na+,K+-ATPase alpha subunit requires gastrulation in the Xenopus embryo

Na+,K+-ATPase participates in reabsorption of ions and water and produces an electrochemical gradient between the intra- and extracellular spaces across the cell membrane. It also plays an important role in many developmental phenomena such as a blastocoele formation and neural formation. To elucidate the expression pattern of Na+,K+-ATPase in the Xenopus embryo, the spatial expression patterns of the Na+,K+-ATPase alpha subunit were studied in a normal embryo by whole-mount in situ hybridization. These transcripts were localized around the dorsal blastopore at the gastrula stage, in the neural tube at the neurula stage, and then in the pronephros and cloaca at the tail-bud stage. To study the function of Na+,K+-ATPase in embryogenesis after mid-blastula transition, the expression of the Na+,K+-ATPase alpha subunit was inhibited by the injection of specific antisense RNA. Embryos injected with Na+,K+-ATPase antisense RNA showed inhibition of gastrulation. When antisense RNA was injected into the dorsal blastomeres, head differentiation was markedly inhibited. These results suggest that this transcript plays an important role during gastrulation and head differentiation.

The role of the cystine loop in acetylcholine receptor assembly

Nicotinic acetylcholine receptors (AChRs) are composed of alpha, beta, gamma, and delta subunits, assembled into alpha2betagammadelta pentamers. A highly conserved feature of ionotropic neurotransmitter receptors, such as AChRs, is a 15-amino acid cystine "loop." We find that an intact cystine loop is necessary for complete AChR assembly. By preventing formation of the loop with 5 mM dithiothreitol, AChR subunits assemble into alphabetagamma trimers, but the subsequent steps in assembly are blocked. When alpha subunit loop cysteines are mutated to serines, assembly is blocked at the same step as with dithiothreitol. In contrast, when beta subunit loop cysteines are mutated to serines, assembly is blocked at a later step, i.e. after assembly of alphabetagammadelta tetramers and before the addition of the second alpha subunit. After formation of the cystine loop, the alpha subunit undergoes a conformational change, which buries the loop. This conformational change is concurrent with the step in assembly blocked by removal of the disulfide bond of the cystine loop, i.e. after assembly of alphabetagamma trimers and before the addition of the delta subunit. The data indicate that the alpha subunit conformational change involving the cystine loop is key to a series of folding events that allow the addition of unassembled subunits.

1,25-Dihydroxyvitamin D3 selectively induces increased expression of the Na,K-ATPase beta 1 subunit in avian myelomonocytic cells without a concomitant change in Na,K-ATPase activity

Treatment of avian myelomonocytic cells with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) results in an approximately two fold increase in levels of Na,K-ATPase beta 1 subunit mRNA and protein (both total and plasma membrane-associated). The changes in beta 1 subunit expression occur in the absence of a detectable increase in expression of any of the three alpha subunit isoforms or in Na,K-ATPase activity. The selective induction of the expression of the beta subunit in avian myelomonocytic cells by 1,25(OH)2D3 reveals a previously unobserved feature of the regulation of Na,K-ATPase expression, while the targeting of beta subunit polypeptides to the plasma membrane in the absence of a corresponding increase in active Na,K-ATPase suggests that, in these cells, transport of the beta subunit to the plasma membrane may be independent of its binding to the alpha subunit.

Subunit interactions in the Na,K-ATPase explored with the yeast two-hybrid system

Subunit interactions of the alpha1- and beta1-subunits of the chicken Na,K-ATPase were explored with the yeast two-hybrid system. Gal4-fusion proteins containing domains of the alpha1- and beta1-subunits were designed for examining both intersubunit and intrasubunit protein-protein interactions. Regions of the alpha- and beta-subunits known to be involved in alpha-beta-subunit assembly were positive in two-hybrid assay, supporting the validity of the assays. A library of beta-subunit ectodomains with C-terminal truncations was screened to find the maximal truncation retaining an interaction with the alpha-subunit extracellular H7H8 loop (where H7 refers to the seventh membrane span, and so on). The maximal truncation removed all the cysteines involved in disulfide bridges, leaving only 63 amino acids of the beta-subunit ectodomain. Scanning alanine mutagenesis led to identification of an evolutionarily conserved sequence of four amino acids (SYGQ) in the extracellular H7H8 loop of the alpha-subunit that is crucial to alpha-beta-intersubunit interactions. Oligomerization studies with single domains failed to detect self-association of either of the two large cytosolic loops (H2H3 and H4H5) within the alpha-subunit. However, evidence was found for an interaction between these two cytoplasmic loops.

Carboxy-terminal regions of the sarcoplasmic/endoplasmic reticulum Ca(2+)- and the Na+/K(+)-ATPases control their K+ sensitivity

The Na+,K(+)-ATPase and the sarcoplasmic/endoplasmic reticulum Ca(2+)-(SERCA-) ATPase belong to a family of P-type ATPases that undergo a cycle of conformational changes between the phosphorylated and dephosphorylated stages in an ion-specific manner. The ouabain-inhibitable Na+,K(+)-ATPase activity requires Na+ and K+. On the other hand, the Ca(2+)-dependent and thapsigargin-inhibitable activity of the SERCA-ATPase does not depend upon Na+ and K+ for its basal activity. However, the SERCA-ATPase and Ca(2+)-transport activities can be further activated either by K+ in a two-step fashion with high (ED50 approximately 20 mM) and low affinity (ED50 approximately 70 mM) or by Na+ in a one-step fashion with an ED50 value of approximately 50 mM. A chimera, in which the carboxy-terminal region (Leu861-COOH) of the Na+,K(+)-ATPase alpha 1 subunit replaced the corresponding region (Ser830-COOH) of the SERCA1-ATPase, lacked the low-affinity K+ activation of the SERCA-ATPase but displayed a higher-affinity (ED50 < 10 mM) activation by K+, similar to that of the Na+,K(+)-ATPase, whereas activation by Na+ was not affected. The replacement of the large cytosolic loop (Gly354-Lys712) and the amino-terminal regions (Met1-Asp162) of the SERCA1-ATPase with the corresponding portions of the Na+,K(+)-ATPase alpha 1 subunit did not affect the sensitivity of the SERCA-ATPase activity to K+. Thus, the carboxy-terminal regions of both the SERCA1 and the Na+,K(+)-ATPase alpha 1 subunit are critical for K+ sensitivity. Analysis of additional (Ca2+/Na+,K+)-ATPase chimeras demonstrated that the carboxy-terminal 102 amino acids (Phe920-Tyr1021) of the Na+/K(+)-ATPase alpha 1 subunit are sufficient to shift the K+ affinity for activation of the SERCA-ATPase without the beta subunit. No change in the two-step activation of SERCA-ATPase by K+ was seen when residues Thr871-Thr898 of the SERCA1-ATPase were replaced with residues Asn894-Ala919 of the Na+,K(+)-ATPase alpha 1 subunit, a region known to bind the Na+,K(+)-ATPase beta subunit [Lemas, M. V., et al. (1994) J. Biol. Chem. 269, 8255-8259]. Thus, the Na+,K(+)-ATPase subunit-assembly domain and the K(+)-sensitive region are distinct within the carboxy-terminal 161 amino acids of the Na+,K(+)-ATPase.

Primary sequence, tissue specificity and mRNA expression of the Na(+),K (+) -ATPase β1 subunit in the European eel (Anguilla anguilla)

The entire amino acid coding sequence of the Na(+),K(+)-ATPase β1 isoform was cloned from the gill of the European eel (Anguilla anguilla) by a PCR based method. The amino acid sequence translated from the nucleotide sequence shared 61.4 and 56.2% homology respectively with previously published Na(+),K(+)-ATPase β1 isoform sequences from the clawed toad (Xenopus laevis) and the ray (Torpedo californica) an elasmobranch fish. The size of the Na(+),K(+)-ATPase β1 mRNA transcript in eel tissues was demonstrated to be 2.35 Kb. Detectable levels of Na(+),K(+)-ATPase β1 mRNA were found at some level in all tissues except liver and cardiac muscle. The level of branchial Na(+),K(+)-ATPase β1 mRNA was observed to increase after the adaptation of fresh water eels to normal or double concentration sea water.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Primary structure of avian H+/K(+)-ATPase beta-subunit

A cDNA encoding a beta-subunit of the avian H+/K(+)-ATPase was cloned from a chicken stomach cDNA library, and its nucleotide sequence determined. A comparison between all the available sequence data for the beta-subunits of P-type ATPases reveals several evolutionarily conserved regions. Overall identity was 66% when compared with mammalian H+/K(+)-ATPase beta-subunits, 34% identity when compared with the Na+/K(+)-ATPase beta 2-subunits, and 33% identity when compared with the Na+/K(+)-ATPase beta 1-subunits.

Structural analysis and expression of a chromosomal gene encoding an avian Na+/K(+)-ATPase beta 1-subunit

Chicken chromosomal DNA encoding the Na+/K(+)-ATPase beta 1-subunit was cloned and characterized. Its exon-intron structure is identical to mammalian (human and rat) beta 1-subunit genes. The transcription initiation site, TATA box, and an ATTGG (antisense CCAAT) sequence follow approximately 1 kilobase of GC-rich 5' upstream sequence that contains many consensus sequences for transcription factors whose relative positions are conserved between human and chicken genes. When this beta 1-subunit gene was stably incorporated into mouse L cells and C2C12 cells, the avian beta 1-subunit was expressed under the control of the its own promoter.

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

The sarcoplasmic reticulum Ca(2+)-ATPase, SERCA1a, contains endoplasmic reticulum targeting information

The fast-twitch skeletal muscle Ca(2+)-ATPase isoenzyme, SERCA1a, is localized in chick skeletal myotubes to both the sarcoplasmic reticulum (SR) and to the nuclear envelope, an extension of the endoplasmic reticulum (ER). The ER labeling remained after cycloheximide treatment, indicating that it did not represent newly synthesized SERCA1a in transit to the SR. Expression of the cDNA encoding SERCA1a in cultured non-muscle cells led to the localization of the enzyme in the ER, as indicated by organelle morphology and the co-localization of SERCA1a with the endogenous ER luminal protein, BiP. Immunopurification analysis showed that SERCA1a was not bound to BiP, nor was any degradation apparent. Thus, the SR Ca(2+)-ATPase appears to contain ER targeting information.

Reversal of the effects of a low extracellular potassium concentration on the number and activity of Na+/K+ pumps in an Epstein-Barr virus-transformed human lymphocyte cell line

A reduction in the extracellular concentration of potassium to 0.5 mM (low K) in Epstein-Barr (EB) virus-transformed lymphocytes caused changes in the number and activity of Na+/K+ pumps in the cell membrane, with increases in the Bmax and apparent Kd of ouabain binding, and concomitant increases in the Vmax and apparent Km of potassium (rubidium) influx. However, recovery from the effects of low K occurred more quickly than the original up-regulation. Furthermore, there were differences in the time-courses of the separate rates of recovery of the Bmax and Kd of ouabain binding after the cells were returned to normal K, the rate of recovery of the Kd being quicker than that of the Bmax, which was biphasic, with slow and fast rates of recovery. Inhibition of protein synthesis by emetine caused an increase in the rate of recovery of the Bmax of ouabain binding, but no effect on the Kd, suggesting that the slow phase of recovery of the Bmax is attributable to the synthesis and insertion of new protein, while the rapid phase of recovery is independent of protein synthesis and may represent internalization. The results suggested that during up-regulation of pump number in response to low K about 40% of the newly inserted Na+/K+ pumps are normal and the rest are abnormal. The half-time of removal of the abnormal pumps from the cell membrane during recovery from low K stress was 2.8 hr and the half-time of internalization of the normal pumps was 4.3 hr.

Synthesis and translocation of Na(+)-K(+)-ATPase alpha- and beta-subunits to plasma membrane in MDCK cells

Synthesis and translocation of Na(+)-K(+)-ATPase alpha-catalytic and beta-glycoprotein subunits from intracellular membranes to the plasma membrane were studied in Madin-Darby canine kidney cells (MDCK-T) by combining the methods of pulse-chase labeling, subcellular fractionation on sorbitol gradients, and immunoprecipitation. Immunoprecipitation from homogenates revealed that radioactive methionine incorporated into beta-subunit was equal to that incorporated into alpha-subunit after 15 min of labeling. Because the ratio of total methionines in alpha- vs. beta-subunit is approximately 5:1, these results suggest that beta-subunit is synthesized in molar excess over alpha-subunit. Half of the newly synthesized beta-subunit, likely unassembled units, were degraded by 60 min after labeling, while alpha-subunits were stable through 120 min after synthesis, suggesting alpha may be limiting for alpha beta-assembly. By 120 min the ratio of counts incorporated into alpha vs. beta approached 5, which is predicted by a 1:1 ratio of alpha to beta. The sorbitol gradient resolved two major membrane samples: a mixture of endoplasmic reticulum and Golgi populations and a plasma membrane-enriched sample. Immature beta (beta i) could not be detected in the plasma membrane-enriched samples at levels greater than could be attributed to cross-contamination by intracellular membranes. Mature beta (beta m) became detectable after 30 min, and conversion of beta i to beta m was 90% complete at 120 min. A peak of labeled alpha-subunit appeared in the plasma membrane-enriched sample at 60 min, coincident with the appearance of labeled beta m-subunit in this sample, suggesting movement as alpha beta-heterodimers.

Expression of Na,K-ATPase isoforms in human heart

The expression pattern of the multiple isoforms of Na,K-ATPase was examined in the human heart. Isoform specific oligonucleotide probes for the alpha 1, alpha 2, alpha 3 and beta 1 subunits were used to probe Northern blots. The adult human ventricle expresses mRNAs for all three alpha subunit isoforms in addition to beta 1 subunit mRNA.

The functional role of the beta-subunit in the maturation and intracellular transport of Na,K-ATPase

The minimal functional enzyme unit of Na,K-ATPase consists of an alpha-beta complex. The alpha-subunit bears all functional domains of the enzyme and so far a regulatory role for the beta-subunit in the catalytic cycle has not been established. On the other hand, increasing experimental evidence suggests that the beta-subunit is an indispensable element for the structural and functional maturation of the enzyme as well as its intracellular transport to the plasma membrane. This brief review summarizes the experimental data supporting the hypothesis that assembly of the beta-subunit is needed for the alpha-subunit to acquire the correct, stable configuration necessary for the acquisition of functional properties and its exit from the ER.

Ouabain- and Ca2(+)-sensitive ATPase activity of chimeric Na- and Ca-pump molecules

Chimeric ion-pumps, consisting of the N-terminal 2/3 of the alpha 1-subunit of the ouabain-sensitive chicken Na+,K(+)-ATPase and the C-terminal 1/3 of the sarcoplasmic reticulum Ca2(+)-ATPase, were expressed in ouabain-insensitive mouse L cells. These chimeric molecules exhibited ouabain-sensitive ATPase activity very similar to that of the wild-type chicken Na+, K(+)-ATPase. This ATPase activity could be stimulated by adding Ca2+ to the assay system. These results suggest that the sites for ouabain-inhibition are restricted to the N-terminal 2/3 of the Na-pump, and the C-terminal 1/3 of the Ca-pump interacts with Ca2+.

Chapter 2 Biogenesis and Sorting of Plasma Membrane Proteins

The cell surface membrane is the boundary between a cell and its environment. In case of polarized epithelial cells, the apical plasma membrane is frequently the boundary between an organism and its environment. The plasmalemma possesses the elements that endow a cell with the capacity to converse with its environment. Plasmalemmal receptor and transducer proteins allow the cell to recognize and respond to various external influences. Membrane-associated proteins anchor cells to their substrata and mediate their integration into tissues. Many properties of a given cell type may be attributed to the protein composition of its plasma membrane. Most cells go to large lengths to control the nature and distribution of polypeptides that populate their plasmalemmas. Cells regulate the expression of genes encoding plasma membrane proteins. Proteins destined for the insertion into the plasma membrane pass through a complex system of processing organelles prior to arriving at their site of ultimate functional residence. Each of these organelles makes a unique contribution to the maturation of these proteins as they transit through them. This chapter discusses the postsynthetic steps involved in the biogenesis of plasma membrane proteins. The chapter discusses some of the events common to all plasmalemmal polypeptides, with special emphasis on those that contribute directly to the character of the cell surface. The chapter then discusses the specializations, associated with cell types, possessing differentiated cell surface sub-domains. The chapter highlights some of the important and fascinating questions confronting investigators interested in the cell biology of the plasma membrane.

Stability of Na(+)-K(+)-ATPase alpha-subunit isoforms in evolution

Encoding DNA for alpha 2- and alpha 3-isoforms of the alpha-subunit of the chicken Na(+)-K(+)-ATPase have been cloned, and their nucleotide sequences and deduced amino acid sequences are reported. Comparisons between these data and comparable data for the rat alpha-subunit isoforms make possible an assessment of alpha-subunit isoform diversity among vertebrates. There is approximately twice as much amino acid sequence difference between alpha-isoforms within a single species as there is difference between corresponding alpha-isoforms of bird and mammal. These data are consistent with triplication of the alpha-subunit gene and evolution of substantially different alpha-subunit isoforms before the separation of avian and mammalian lineages over 200 million years ago and then retention of the majority of these structural differences through subsequent evolution. The implications of this conversation of isoform-specific structural features are discussed in terms of transport functions and bioregulation of the Na(+)-K(+)-ATPase.

Molecular cloning of the beta-subunit of the Na,K-ATPase in the brine shrimp, Artemia. The cDNA-derived amino acid sequence shows low homology with the beta-subunits of vertebrates except in the single transmembrane and the carboxy-terminal domains

A cDNA encoding the beta-subunit of the Na,K-ATPase of brine shrimp (Artemia) has been cloned. Its nucleotide sequence and predicted amino acid sequence have been determined. The amino acid sequence shows considerable divergence from that of chicken, dog, human, pig, rat, sheep, Torpedo, and Xenopus. This is not entirely unexpected since brine shrimp is a 'fast clock' organism which diverged from the precursor of the vertebrates 0.5-1.0 billion years ago. However, a highly hydrophobic putative transmembrane domain and the carboxy-terminal domain show considerable conservation. The relatively small degree of conservation in the beta-subunit of Artemia should provide information about the functional significance of this protein.

pOEV: a Xenopus oocyte protein expression vector

We have constructed a Xenopus laevis oocyte expression vector, pOEV, which allows cloned DNA to be transcribed and translated directly in the oocyte. Since proteins translated in oocytes are post-translationally modified according to conserved eukaryotic signals, these cells offer a convenient system for performing structural and functional analyses of cloned genes. pOEV can be used for direct analysis of proteins encoded by cloned cDNAs without preparing mRNA in vitro, simplifying existing protocols for translating proteins in oocytes with a very high translational yield. Transcription of the vector in oocytes is driven by the promoter for the TFIIIA gene, which can generate 1-2 ng (per oocyte within 2 days) of stable mRNA template for translation. The vector also contains SP6 and T7 promoters for in vitro transcription to make mRNA and hybridization probes. DNA clones encoding chloramphenicol acetyltransferase (CAT) were injected into oocyte germinal vesicles and CAT protein accumulated in the cell over a 2- to 4-day period. We found that the concentration of DNA injected affected protein yields; surprisingly relatively low concentrations in the range 25-50 pg DNA per oocyte gave maximum yields of CAT protein. When as little as 5 pg of pOEV DNA is injected we typically expressed 40 fmol of CAT protein per oocyte, after 4-day incubations. In addition, we have shown that this system is amenable to the expression of nuclear and membrane proteins.

Na(+)-K(+)-ATPase gene expression during in vitro development of rat fetal forebrain

The existence of at least three isoforms of Na(+)-K(+)-ATPase in adult brain tissues [alpha 1, kidney type; alpha 2 [or alpha(+)]; alpha 3] suggests that these genes might be regulated in a cell-specific and time-dependent manner during development. We have studied this question in serum-free aggregating cell cultures of mechanically dissociated rat fetal telencephalon. At the protein level, the relative rate of synthesis of the pool of alpha 1-, alpha 2-, and alpha 3-subunits increased approximately twofold over 15 days of culture, leading to a marked increase in the immunochemical pool of alpha-subunits as measured by a panspecific polyclonal antibody. Concomitantly, Na(+)-K(+)-ATPase enzyme-specific activity increased three- (lower forebrain) to sixfold (upper forebrain). The transcripts of all three alpha-isoforms and beta-subunit were detected in vitro in similar proportion to the level observed in vivo. alpha 3-mRNA (3.7 kb) was more abundant than alpha 1 (3.7 kb) or alpha 2 (5.3 and 3.4 kb). Cytosine arabinoside (0.4 microM) and cholera toxin (0.1 microM) were used to selectively eliminate glial cells or neurons, respectively. It was found that alpha 2-mRNA is predominantly transcribed in glial cell cultures, whereas alpha 3- and beta 1-mRNA (2.7, 2.3, and 1.8 kb) are predominant in neuronal cultures.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Transmembrane segments of the P-type cation-transporting ATPases. A comparative study

The transmembrane segments predicted for the Neurospora H-ATPase are laid out diagrammatically in Figure 10. Although the eight segments have arbitrarily been compressed into rectangles of the same size, they range in length from 20 residues (II) to 30 residues (IV and VI), so the corresponding helices must vary in length as well. Notable features of the model include the charged residues located just outside the plane of the membrane, with a clear excess of negative charges (5-, 1+) at the extracellular surface and a slight excess of positive charges (4+, 3-) at the cytoplasmic surface. There are also a conspicuous number of bulky residues (tryptophan, phenylalanine, and tyrosine) just inside the plane of the membrane. Within the bilayer, most of the helices are noticeably amphipathic, consistent with the expectation that at least some of them stack together to form a channel-like structure with a hydrophobic surface and a hydrophilic core. The charged residues predicted to lie within the membrane are listed in Table 2, which is a summary of data from eight of the P-type ATPases; the S. cerevisiae and S. pombe enzymes have not been included because they are nearly identical in this respect to the Neurospora enzyme. Interestingly, all of the ATPases have more membrane-embedded negative charges (5 to 8) than positive ones (0 to 4), a pattern that may be connected with their role as cation transporters. Certainly, other unrelated transport proteins have a rather different pattern of positive and negative charges: for example, the mammalian glucose transporter (1+, 2-), Na-glucose transporter (3+, 3-), and the E. coli lac permease (11+, 7-). The actual positioning of the negative charges in the P-type ATPases does not make it easy to single out the functionally important ones, however. The glutamyl residue in segment I is present in the fungal, plant, and Leishmania H-ATPases but not in the gastric H,K-ATPase. The same is true for the aspartate in segment II, except that it also appears in the muscle and brain Ca-ATPases. A glutamate is found at one end of segment III in the E. coli and fungal enzymes and at the other end in Arabidopsis; in segment IV, another glutamate appears in a well-conserved region in the Leishmania and mammalian enzymes but not in the bacterial, fungal, or plant ones.(ABSTRACT TRUNCATED AT 400 WORDS)

A role for the beta-subunit in the expression of functional Na+-K+-ATPase in Xenopus oocytes

In all cellular systems studied so far, the catalytic alpha- and the glycosylated beta-subunit of Na+-K+-ATPase are coordinately synthesized and are assembled into stoichiometric alpha, beta-complexes. In contrast to these data, in this study we show that the fully grown oocyte of Xenopus laevis synthesizes much less beta-subunit than alpha-subunit. The alpha-subunit produced in excess over the beta-subunit is membrane associated but highly trypsin sensitive and can be compared with the immature alpha-subunit population identified in epithelial cells immediately after synthesis (K. Geering, J. P. Kraehenbuhl, and B.C. Rossier, J. Cell Biol. 105: 2613-2619, 1987). The Xenopus oocyte thus turns out to be a unique system to study the functional role of the beta-subunit. Injection of beta-subunit-specific mRNA transcribed in vitro from a beta-cDNA clone (derived from Xenopus kidney, A6 cells) into oocytes results in translation of a glycosylated beta-subunit. The synthesis of this exogenous beta-subunit increases significantly the proportion of trypsin-resistant oocyte alpha-subunits able to perform cation-dependent conformational changes. In addition, 25-65% more ouabian binding sites are expressed at the plasma membrane in beta-mRNA-injected oocytes. In contrast, newly synthesized alpha-subunit translated after injection of size-fractionated mRNA enriched in alpha-mRNA remains trypsin sensitive as the oocyte alpha-subunit. These data suggest that association of the beta-subunit to the alpha-subunit provokes a structural rearrangement of the alpha-subunit that might be a first step toward the functional maturation of the Na+-K+-ATPase and its expression at the plasma membrane.