Biomedical subjects

R A Schwalbe

Publications and source records attributed to R A Schwalbe.

10 recordsLinked to original sources

PubMed1998

Functional consequences of ROMK mutants linked to antenatal Bartter's syndrome and implications for treatment.

The antenatal variant of Bartter's syndrome is an autosomal recessive kidney disease characterized by polyhydramnios, premature delivery, hypokalemic alkalosis and hypercalciuria. It is genetically heterogeneous, having been linked recently to mutations in an ATP-sensitive, renal outer medullary K+channel, ROMK, and earlier to mutations in the Na-K-2Cl co-transporter, NKCC2. We characterized four of the mutations reported in three heterozygous ROMK variants of antenatal Bartter's and found that each expressed a distinct phenotype in Sf9 cells. One mutation expressed normal function and appears to be an allelic polymorphism. The other three mutations produced channels with significantly reduced K+fluxes. However, the mechanisms in each case were different and reflected abnormalities in phosphorylation, proteolytic processing or protein trafficking. The different mechanisms may be important in the design of appropriate therapy for patients with this disease.

Animals↗

PubMed1997

Mapping the kidney potassium channel ROMK1. Glycosylation of the pore signature sequence and the COOH terminus.

ROMK1, also known as Kir 1.1, is an inwardly rectifying K+ channel and is the prototypical member of the large Kir gene family. The accepted model of Kir topology predicts intracellular NH2 and COOH termini, and two membrane-spanning segments, M1 and M2, connected by an intramembranous pore-forming segment, H5. The sequence of H5 is similar in voltage-dependent K+ channels and features a strictly conserved GY/FG in its mid-region, which has been proposed as the selectivity filter of the pore. We have been using N-glycosylation substitution mutants to map the extracellular topology of ROMK1 biochemically and have described several loci in H5 that were glycosylated. We now report glycosylation at loci Tyr144 and Phe146, which indicates that the signature GYG sequence (143-145) rather than being intramembranous is extracellular. The COOH terminus was predicted to begin at position 178, but contrary to the model, we observed that position 257 was glycosylated and surrounding positions at 199, 222, and 298 were unglycosylated. N-Glycosylation sequon substitution at the latter three positions abolished K+/Na+ selectivity. Our results suggest a major revision of the topology of ROMK1 with H5 and the pore signature sequence now completely extracellular. The COOH terminus appears to form two additional membrane-spanning segments and to contribute to the ion conduction pathway.

Amino Acid Sequence↗

PubMed1997

Comparison of naturally occurring vitamin K-dependent proteins: correlation of amino acid sequences and membrane binding properties suggests a membrane contact site.

Membrane-binding properties of human and bovine forms of vitamin K-dependent proteins Z, S, and C were characterized. Each of these proteins showed unique properties and interspecies differences that correlated with specific amino acid sequence variations in the amino-terminal 45 residues. Protein Z showed 100-fold slower membrane binding and dissociation kinetics relative to other vitamin K-dependent proteins that have been tested. This property seemed to correlate with an extra gamma-carboxyglutamic acid (Gla) residue at position 11 of protein Z. The interspecies difference for protein Z consisted of a higher packing density for the bovine protein on the membrane and a 9-fold slower dissociation rate. Higher affinity correlated with Asp at position 34 of bovine protein Z, where the human protein contains Asn. While both protein S species showed high affinity for the membrane, it was significantly greater for the human protein versus bovine protein S. Again, higher affinity correlated with an Asp (vs Asn) at position 34. Protein C was characterized by binding affinities that were 100-1000-fold lower than the other proteins. Low affinity appeared to be related to loss of Gla-32 (homologous to Gla-33 of protein Z). Interspecies differences of protein C appeared to be related to proline at position 10 (homologous to position 11 of protein Z) of bovine protein C, which produced at least 10-fold lower affinity than the human protein. Comparable substitutions at positions homologous to 11, 33, and 34 of protein Z may also underlie membrane binding behaviors of other vitamin K-dependent proteins. The three-dimensional structure of strontium-prothrombin fragment 1 [Seshadri et al. (1994) Biochemistry 33, 1087] shows that these positions are clustered on the protein surface near strontium-8, another possible candidate for membrane contact. A membrane contact mechanism consisting of an isolated protein-lipid ion pair is proposed. Comparison of naturally occurring vitamin K-dependent proteins has provided possible bases for divergent membrane binding and suggested future approaches to determine biological function.

Amino Acid Sequence↗

PubMed1996

Novel sites of N-glycosylation in ROMK1 reveal the putative pore-forming segment H5 as extracellular.

Inwardly rectifying K+ channels (IRKs) maintain resting membrane potential, excitability, and K+ exchange. The proposed topological model of IRKs consists of intracellular amino and carboxyl termini and two transmembrane segments (M1 and M2) linked by a pore-forming segment (H5). Structure-function studies have identified critical pore determinants in M2 and the carboxyl terminus but not as expected by analogy with voltage-dependent K+ channels, in H5. We investigated the topology of the IRK ROMK1 by substituting novel N-glycosylation sites which act as markers for extracellular segments. N-Glycosylation, before and after an N-glycosylation inhibitor, tunicamycin, was measured directly by gel shift assays and changes in membrane currents. Tunicamycin produced gel shifts and changes in membrane currents that correlated exactly. N-Glycosylation sites substituted into the amino and carboxyl termini and the M1 segment gave results consistent with the proposed model. N-Glycosylation sites were distributed throughout H5 and its flanking regions indicating that H5 is mainly extracellular. Thus, the linker between M1 and M2 has little or no intramembranous component.

Animals↗

PubMed1995

Association of rat C-reactive protein and other pentraxins with rat lipoproteins containing apolipoproteins E and A1.

C-Reactive protein (CRP) is a member of the pentraxin family of proteins, ubiquitous components of animal serum. This study suggests that, in serum, rat CRP is complexed with lipoprotein and may interact directly with apolipoprotein E. When mixed with diluted rat serum, radiolabeled rat CRP showed a slightly higher sedimentation coefficient (about 15%) than that of the free protein. Elimination of calcium or addition of O-phosphorylethanolamine (O-PE), a low molecular weight compound that binds tightly to rat CRP in a calcium-dependent manner, abolished this difference. Adsorption of rat serum on a rat CRP affinity gel and elution with PE resulted in the isolation of material containing high levels of apolipoproteins E and A1. The affinity-purified preparation interacted with rat CRP and altered the sedimentation coefficient of the latter to the value observed in whole serum. Conversely, rat CRP increased the sedimentation coefficient of the major component of the affinity-purified material or to diluted rat serum, human serum amyloid P (SAP) and hamster female protein (FP), two other members of the pentraxin protein family, also had slightly higher sedimentation coefficients. In contrast, human CRP showed no evidence of an interaction in rat serum or with the affinity-purified proteins. This selectivity coincided with the ability of these pentraxins to bind to O-PE with high affinity. The sedimentation properties of serum lipoproteins, radiolabeled with [3H]cholesterol, also suggested an interaction with rat CRP.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-Globulins↗

PubMed1995

Potassium channel structure and function as reported by a single glycosylation sequon.

Inwardly rectifying K+ channels (IRKs) are highly K(+)-selective, integral membrane proteins that help maintain resting the membrane potential and cell volume. Integral membrane proteins as a class are frequently N-glycosylated with the attached carbohydrate being extracellular and perhaps modulating function. However, dynamic effects of glycosylation have yet to be demonstrated at the molecular level. ROMK1, a member of the IRK family is particularly suited to the study of glycosylation because it has a single N-glycosylation consensus sequence (Ho, K., Nichols, C. G., Lederer, W. J., Lytton, J., Vassilev, P. M., Kanazirska, M. V., and Herbert, S. C. (1993) Nature 362, 31-38). We show that ROMK1 is expressed in a functional state in the plasmalemma of an insect cell line (Spodoptera frugiperda, Sf9) and has two structures, glycosylated and unglycosylated. To test functionality, glycosylation was abolished by an N117Q mutation or by treatment with tunicamycin. Whole cell currents were greatly reduced in both of the unglycosylated forms compared to wild-type. Single channel currents revealed a dramatic decrease in opening probability, po, as the causative factor. Thus we have shown biochemically that the N-glycosylation sequon is extracellular, a result consistent with present topological models of IRKs, and we conclude that sequon occupancy by carbohydrate stabilizes the open state of ROMK1.

Amino Acid Sequence↗

PubMed1992

Pentraxin family of proteins interact specifically with phosphorylcholine and/or phosphorylethanolamine.

Pentraxins are a family of serum proteins characterized by five identical subunits that are noncovalently linked. The two major types of pentraxins are C-reactive protein (CRP) and serum amyloid P component (SAP). CRP proteins are identified by their calcium-dependent interaction with phosphorylcholine. This study showed that SAP also bound to phosphorylated compounds but had a high specificity for phosphorylethanolamine. Thus, human CRP and SAP show high specificity that is complementary for the related compounds, phosphorylcholine and phosphorylethanolamine, respectively. This relationship suggests a complementary and/or related function for the pentraxins. Pentraxins from other species were also examined. Mouse SAP showed binding interactions and specificity similar to human SAP. Female protein (FP) from hamster and rat CRP showed a hybrid specificity and bound to both phosphorylethanolamine and phosphorylcholine. All of the proteins that bound phosphorylethanolamine also associated with human C4b-binding protein (C4BP). With the exception of human and rat CRP, all the proteins also bound to vesicles containing acidic phospholipids. All of these binding interactions were calcium-dependent and mutually exclusive, suggesting that they involved the same site on the protein. These findings suggest possible ways to examine the function of the pentraxins.

Alpha-Globulins↗

PubMed1991

Heparin influence on the complex of serum amyloid P component and complement C4b-binding protein.

Serum amyloid P component (SAP) forms a calcium-dependent complex with C4b-binding protein (C4BP) in human serum. this study demonstrated that heparin interacted with SAP in a calcium-dependent manner and prevented formation of the SAP.C4BP complex. Furthermore, the SAP-heparin interaction interfered with SAP binding to membranes. Therefore, all three of these interactions involved similar sites on SAP, or each interaction sterically obstructed the other binding sites. In addition to heparin, SAP bound to heparan sulfate and chondroitin sulfate. In each case, a distinct multimeric species was generated. Gel filtration and sucrose density gradient ultracentrifugation suggested that heparin and heparan sulfate produced a dimer of SAP. The dimer appeared to be the most stable structure since it was not dissociated by excess heparin. While low molecular weight heparin interacted with SAP and inhibited SAP association with membranes, the SAP dimer was not detected in sucrose density gradient ultracentrifugation studies. Polybrene prevented the interaction between SAP and heparin in both a purified system and in human serum that was enriched in SAP and heparin. In contrast, Polybrene did not seem to alter the SAP.C4BP complex. While the function of the SAP.C4BP complex is unknown, it may be important for regulation of complement and/or transport of SAP to sites in the body. Dissociation of the SAP.C4BP complex by sulfated polysaccharides such as heparin may be a physiological response that could be important during tissue damage or complement activation.

Carrier Proteins↗

PubMed1990

Independent association of serum amyloid P component, protein S, and complement C4b with complement C4b-binding protein and subsequent association of the complex with membranes.

C4b-binding protein (C4BP) is a large complex assembly of eight subunits that functions as an inhibitor of the complement cascade. A portion of the C4BP in serum exists as a complex with protein S. This study demonstrated that another protein, serum amyloid P component (SAP), also formed a calcium-dependent complex with C4BP. The C4BP.SAP complex was detected by several methods including light scattering intensity, gel filtration, and sucrose density gradient ultracentrifugation. This complex was of high affinity relative to serum levels of these proteins so that no dissociation was detected at 3% of serum protein concentrations. The C4BP.SAP complex was also detected in normal serum and the results suggested that there was virtually no free SAP or uncomplexed C4BP in normal serum. In addition to its complex with C4BP, SAP underwent other calcium-dependent associations such as binding to phospholipid vesicles and self-aggregation. Self-aggregation was highly cooperative with kinetics corresponding to a reaction that was 6th-order with respect to calcium and required about 1.5 mM calcium. In contrast, formation of the SAP.C4BP complex and interaction of SAP with membranes required only about 0.4 and 1.0 mM calcium, respectively. Thus, selection of the correct conditions allowed study of the SAP.C4BP interaction without interference from self-aggregation. All three of these interactions of SAP were mutually exclusive and the SAP. C4BP interaction appeared to be favored over self-aggregation or binding of SAP to phospholipids. It seems likely that the biologically dominant interaction for SAP is with C4BP. The SAP.C4BP complex interacted with protein S and these binding sites appeared to be entirely independent. Furthermore, SAP had little or no effect on the ability of C4BP to bind C4b. Finally, the entire complex of proteins (C4BP, SAP, protein S, and C4b) could associate with membranes in the presence of calcium. Membrane binding occurred through the protein S component. This rather complicated assemblage of proteins probably functions in a regulatory role for the complement cascade or other biological systems. It is possible that elevated levels of SAP or nonequivalent levels of SAP and C4BP could contribute to certain pathological conditions.

Calcium↗

PubMed1989

Protein structural requirements and properties of membrane binding by gamma-carboxyglutamic acid-containing plasma proteins and peptides.

The membrane-binding characteristics of a number of modified vitamin K-dependent proteins and peptides showed a general pattern of structural requirements. The amino-terminal peptides from human prothrombin (residues 1-41 and 1-44, 60:40) bovine factor X (residues 1-44), and bovine factor IX (residues 1-42), showed a general requirement for a free amino-terminal group, an intact disulfide, and the tyrosine homologous to Tyr44 of factor X for membrane binding. Consequently, the peptide from factor IX did not bind to membranes. Any of several modifications of the amino terminus, except reaction with trinitrobenzenesulfonic acid, abolished membrane binding by the factor X and prothrombin peptides. Calcium, but not magnesium, protected the amino terminus from chemical modification. The requirement for a free amino terminus was also shown to be true for intact prothrombin fragment 1, factor X, and factor IX. Although aggregation of the peptide-vesicle complexes greatly complicated accurate estimation of equilibrium binding constants, results with the factor X peptide indicated an affinity that was not greatly different from that of the parent protein. The most striking difference shown by the peptides was a requirement for about 10 times as much calcium as the parent proteins. In a manner similar to the parent proteins, the prothrombin and factor X peptides showed a large calcium-dependent quenching of tryptophan fluorescence. This fluorescence quenching in the peptides also required about 10 times the calcium needed by the parent proteins. Thus, the 1-45 region of the vitamin K-dependent proteins contained most of the membrane-binding structure but lacked component(s) needed for high affinity calcium binding. Protein S that was modified by thrombin cleavage at Arg52 and Arg70 showed approximately the same behavior as the amino-terminal 45-residue peptides. That is, it bound to membranes with overall affinity that was similar to native protein S but required high calcium concentrations. These results suggested that the second disulfide loop of protein S (Cys47-Cys72) and prothrombin (Cys48-Cys61) were involved in high affinity calcium binding. Since factor X lacks a homologous disulfide loop, an alternative structure must serve a similar function. A striking property of protein S was dissociation from membranes by high calcium. While this property was shared by all the vitamin K-dependent proteins, protein S showed this most dramatically and supported protein-membrane binding by calcium bridging.

1-Carboxyglutamic Acid↗