Phase separation of rat intestinal brush border membrane proteins using Triton X-114

Identification and characterization of two distinct ligand binding regions of cubilin

Using polymerase chain reaction-amplified fragments of cubilin, an endocytic receptor of molecular mass 460 kDa, we have identified two distinct ligand binding regions. Region 1 of molecular mass 71 kDa, which included the 113-residue N terminus along with the eight epidermal growth factor (EGF)-like repeats and CUB domains 1 and 2, and region 2 of molecular mass 37 kDa consisting of CUB domains 6-8 bound both intrinsic factor-cobalamin (vitamin B(12); Cbl) (IF-Cbl) and albumin. Within these two regions, the binding of both ligands was confined to a 110-115-residue stretch that encompassed either the 113-residue N terminus or CUB domain 7 and 8. Ca(2+) dependence of ligand binding or the ability of cubilin antiserum to inhibit ligand binding to the 113-residue N terminus was 60-65%. However, a combination of CUB domains 7 and 8 or 6-8 was needed to demonstrate significant Ca(2+) dependence or inhibition of ligand binding by cubilin antiserum. Antiserum to EGF inhibited albumin but not IF-Cbl binding to the N-terminal cubilin fragment that included the eight EGF-like repeats. While the presence of excess albumin had no effect on binding to IF-Cbl, IF-Cbl in excess was able to inhibit albumin binding to both regions of cubilin. Reductive alkylation of the 113-residue N terminus or CUB 6-8, CUB 7, or CUB 8 domain resulted in the abolishment of ligand binding. These results indicate that (a) cubilin contains two distinct regions that bind both IF-Cbl and albumin and that (b) binding of both IF-Cbl and albumin to each of these regions can be distinguished and is regulated by the nonassisted formation of local disulfide bonds.

Caco-2 cells express a combination of colonocyte and enterocyte phenotypes

Caco-2 cells are derived from a human colonic adenocarcinoma, but differentiate into small intestinal-like cells after confluence. While this enterocytic differentiation has been well studied, the presumed parallel loss of colonocyte function has not been as thoroughly examined. To follow the phenotype for both tissues, Western blots were performed using antisera recognizing liver/bone/kidney alkaline phosphatase and surfactant-like particle proteins found in normal human colon, along with antisera against the small bowel representatives of the same proteins. Antisera against proteins enriched in either enterocytes (alpha1-antitrypsin) or colonocytes (surfactant protein A) were also evaluated. Alkaline phosphatase activity increased from 3 to 18 days post-confluence. Activity at 3 days post-confluence derived substantially from both isomers. Thereafter, the colonic (liver/bone/kidney) isomer declined to low levels as the content of the enterocytic isomer rose. A similar pattern was found with colonic (decreasing expression) and enterocytic (increasing expression) surfactant-like particle proteins. In particular, the content of larger enterocytic particle proteins (97 and 116 kDa) increased with time in culture. Expression of alpha1-antitrypsin increased early and remained high, whereas surfactant protein A generally declined after the third day post-confluency. In summary, undifferentiated Caco-2 cells express very low levels of proteins characteristic of either colonocytes or enterocytes. Immediately after confluence, they expressed proteins characteristic of both cell types. Thereafter, the content of colonocyte-specific proteins decreased, whereas those specific for the enterocyte increased. The timing and degree of this phenotypic switch have implications for the interpretation of experiments using Caco-2 cells as a model of small intestinal function.

Pervanadate activation of intracellular kinases leads to tyrosine phosphorylation and shedding of syndecan-1

Syndecan-1 is a transmembrane haparan sulphate proteoglycan that binds extracellular matrices and growth factors, making it a candidate to act between these regulatory molecules and intracellular signalling pathways. It has a highly conserved transmembrane/cytoplasmic domain that contains four conserved tyrosines. One of these is in a consensus sequence for tyrosine kinase phosphorylation. As an initial step to investigating whether or not phosphorylation of these tyrosines is part of a signal-transduction pathway, we have monitored the tyrosine phosphorylation of syndecan-1 by cytoplasmic tyrosine kinases in intact cells. Tyrosine phosphorylation of syndecan-1 is observed when NMuMG cells are treated with sodium orthovanadate or pervanadate, which have been shown to activate intracellular tyrosine kinases. Initial studies with sodium orthovanadate demonstrate a slow accumulation of phosphotyrosine on syndecan-1 over the course of several hours. Pervanadate, a more effective inhibitor of phosphatases, allows detection of phosphotyrosine on syndecan-1 within 5 min, with peak phosphorylation seen by 15 min. Concurrently, in a second process activated by pervanadate, syndecan-1 ectodomain is cleaved and released into the culture medium. Two phosphorylated fragments of syndecan-1 of apparent sizes 6 and 8 kDa remain with the cell after shedding of the ectodomain. The 8 kDa size class appears to be a highly phosphorylated form of the 6 kDa product, as it disappears if samples are dephosphorylated. These fragments contain the C-terminus of syndecan-1 and also retain at least a portion of the transmembrane domain, suggesting that they are produced by a cell surface cleavage event. Thus pervanadate treatment of cells results in two effects of syndecan-1: (i) phosphorylation of one or more of its tyrosines via the action of a cytoplasmic kinase(s) and (ii) cleavage and release of the ectodomain into the medium, producing a C-terminal fragment containing the transmembrane/cytoplasmic domain.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Glycerol allows low-temperature phase separation of membrane proteins solubilized in Triton X-114: application to the purification of plant cytochromes P-450 and b5

The potentiality of the Triton X-114 phase separation technique for the purification of proteins from plant microsomal membranes has been investigated. It was shown that glycerol significantly lowers the cloud point of Triton X-114 solutions in water and of Triton X-114 solubilizates from microsomal membranes. It was also established that solubilized membrane components decrease the temperature of Triton X-114 micellar aggregation. Solubilization of microsomal membrane using detergent to protein ratios lower than 3.5, however, resulted in complete inhibition of detergent phase separation. Phase partitioning of Triton X-114 microsomal solubilizates, performed at low temperature (4 degrees C), in the presence of glycerol, provided a very fast and efficient step for the purification of cytochromes P-450 and b5. Conditions allowing optimal recoveries of these cytochromes have been defined.

Purification and immunocharacterization of a plant cytochrome P450: the cinnamic acid 4-hydroxylase

Cinnamic acid 4-hydroxylase (CA4H) was purified from microsomes of manganese-induced Jerusalem artichoke (Helianthus tuberosus L.) tuber tissues. The three-step purification procedure involved solubilization and phase partitioning in Triton X-114, followed by chromatography on DEAE-Trisacryl and hydroxylapatite columns. Purification was monitored using carbon monoxide and type I substrate binding properties of the enzyme. The protein, purified to electrophoretic homogeneity, showed an Mr of about 57,000 on SDS-PAGE. Polyclonal antibodies raised against this protein selectively reacted with a 57-kDa polypeptide on Western blots of induced Jerusalem artichoke microsomes. The antibody selectively and strongly inhibited CA4H activity from several plant species.

Quantitation and topography of membrane proteins in highly water-permeable vesicles from ADH-stimulated toad bladder

Antidiuretic hormone (ADH) stimulation of toad bladder granular cells rapidly increases the osmotic water permeability (Pf) of their apical membranes by insertion of highly selective water channels. Before ADH stimulation, these water channels are stored in large cytoplasmic vesicles called aggrephores. ADH causes aggrephores to fuse with the apical membrane. Termination of ADH stimulation results in prompt endocytosis of water channel-containing membranes via retrieval of these specialized regions of apical membrane. Protein components of the ADH water channel contained within these retrieved vesicles would be expected to be integral membrane protein(s) that span the vesicle's lipid bilayer to create narrow aqueous channels. Our previous work has identified proteins of 55 (actually a 55/53-kDa doublet), 17, 15, and 7 kDa as candidate ADH water channel components. We now have investigated these candidate ADH water channel proteins in purified retrieved vesicles. These vesicles do not contain a functional proton pump as assayed by Western blots of purified vesicle protein probed with anti-H(+)-ATPase antisera. Approximately 60% of vesicle protein is accounted for by three protein bands of 55, 53, and 46 kDa. Smaller contributions to vesicle protein are made by the 17- and 15-kDa proteins. Triton X-114-partitioning analysis shows that the 55, 53, 46, and 17 kDa are integral membrane proteins. Vectorial labeling analysis with two membrane-impermeant reagents shows that the 55-, 53-, and 46-kDa protein species span the lipid bilayer of these vesicles. Thus the 55-, 53-, and 46-kDa proteins possess characteristics expected for ADH water channel components. These data show that the 55- and 53- and perhaps the 46-, 17-, and 15-kDa proteins are likely components of aqueous transmembrane pores that constitute ADH water channels contained within these vesicles.

Rat intestinal alkaline phosphatase secretion into lumen and serum is coordinately regulated

We have reported the presence of intestinal alkaline phosphatase on particles with surfactant-like properties within enterocytes, on the luminal surface (light mucosal scrapings) and in the lumen of adult fat-fed rat intestines ((1989) J. Clin. Invest. 84, 1355). To test the physiological role of these particles, we compared the effect on particle secretion of a known inducer of luminal and serum alkaline phosphatase secretion (fat), with the effect of pharmacological stimulators (cholecystokinin and bethanecol). Fat induced a 2-3-fold increase in membrane-free phosphatase activity in serum, and in particle-bound alkaline phosphatase activity in proximal luminal washings and light mucosal scrapings, reaching a peak in both compartments 7 h after a corn oil feed. Bethanecol given subcutaneously induced a quantitatively similar increase in serum alkaline phosphatase activity and in particle-bound phosphatase activity in proximal light mucosal scrapings, reaching a peak 7.5 min after injection. Cholecystokinin also had a 2-3-fold stimulatory effect, 30 min after injection, on particle-bound phosphatase activity in proximal intestinal light mucosal scrapings and distal intestinal luminal washings. The increase in alkaline phosphatase activity in serum samples reached a peak 60 min after cholecystokinin injection. Thus, three independent stimuli increase both luminal and serum appearance of intestinal alkaline phosphatase. These data support the earlier findings that intestinal alkaline phosphatase secretion into the lumen is mediated by a secreted particle, further show that secretion into serum and lumen is coordinately regulated, and are consistent with the hypothesis that the rise in serum alkaline phosphatase activity could be related to extracellular release of the enzyme from the particles.

Association of the intestinal brush-border membrane phospholipase A2 and lysophospholipase activities (phospholipase B) with a stalked membrane protein

We have attempted to determine the size and membrane orientation of a recently described rat jejunal brush-border protein possessing phospholipase A2 and lysophospholipase activities (phospholipase B) (Pind, S. and Kuksis, A. [1988] Biochim, Biophys. Acta 938, 211-221). The phospholipase A2 and lysophospholipase activities were renatured following nonreducing sodium dodecyl sulphate polyacrylamide gel electrophoresis of the total membrane proteins and were shown to migrate as a component of a protein band having a relative molecular mass of 170 kDa. This band accounted for approximately 1% of the total Coomassie Blue staining proteins. Phospholipase B was also shown to be solubilized from the membranes, in an active form, by a proteolytic digestion with papain. Papain solubilization resulted in a loss of the hydrophobic properties observed for the intact phospholipase. These results suggest that the active site of the phospholipase projects from the luminal surface of the membrane vesicles. In support of this, phospholipase activity towards exogenous, detergent-solubilized phosphatidylcholine was demonstrated under conditions in which the membranes remained intact. We conclude that the phospholipase B has the characteristics of a stalked, brush-border membrane protein and may be considered as another digestive enzyme anchored in this membrane.

Specific alterations of rat renal microsomal proteins induced by cephaloridine

In order to elucidate the molecular mechanisms of cephaloridine (CPH) nephrotoxicity, the effect of cephaloridine treatment on the protein composition of different subcellular fractions from rat kidney cortex was investigated. After intravenous treatment of male Wistar rats with 250-1200 mg/kg/d CPH for 1-3 days, kidneys were removed and the homogenate from renal cortex was separated into lysosomal, cytosolic and microsomal fractions. The polypeptide composition of the different subfractions was analyzed by one-dimensional SDS-gel electrophoresis and quantified by densitometry. Significant differences in the polypeptide composition between treated and non-treated animals were seen in the microsomal fraction. CPH-treatment induced a polypeptide with an apparent molecular weight of 44,000 and decreased the content of cytochrome P-450 isoenzymes in the microsomal fraction. Solubilization experiments showed that the CPH-induced microsomal polypeptide of molecular weight 44,000 is a peripheral membrane protein rather than an integral membrane protein. The induction of this protein by CPH was dose- and time-dependent. Preliminary experiments using the kidney slice technique indicate that the induction of this polypeptide correlates with the nephrotoxicity measured as decrease in renal cortical accumulation of organic ions. Thus, the results of the present study indicate that treatment of rats with CPH resulted in the induction of a microsomal polypeptide of molecular weight 44,000 which could be a sensitive parameter of cephaloridine nephrotoxicity.

Studies on protein kinases involved in regulation of nucleocytoplasmic mRNA transport

The rate of energy-dependent nucleoside triphosphatase (NTPase)-mediated nucleocytoplasmic translocation of poly(A)-containing mRNA [poly(A)+mRNA] across the nuclear envelope is thought to be regulated by poly(A)-sensitive phosphorylation and dephosphorylation of nuclear-envelope protein. Studying the phosphorylation-related inhibition of the NTPase, we found that phosphorylation of one polypeptide of rat liver envelopes by endogenous NI- and NII-like protein kinase was particularly sensitive to poly(A). This polypeptide (106 kDa) was also phosphorylated by nuclear-envelope-bound Ca2+-activated and phospholipid-dependent protein kinase (protein kinase C). Activation of kinase C by tumour-promoting phorbol esters resulted in inhibition of nuclear-envelope NTPase activity and in a concomitant decrease of mRNA (actin) efflux rate from isolated rat liver nuclei. Protein kinase C, but not nuclear envelope NI-like or NII-like protein kinase, was found to be solubilized from the envelope by Triton X-100, whereas the presumable poly(A)-binding site [the 106 kDa polypeptide, representing the putative carrier for poly(A)+mRNA transport] remained bound to this structure. RNA efflux from detergent-treated nuclei lost its susceptibility to phorbol esters. Addition of purified protein kinase C to these nuclei restored the effect of the tumour promoters. Protein kinase C was found to bind also to isolated rat liver nuclear matrices in the absence but not in the presence of ATP. The NII-like nuclear-envelope protein kinase co-purified together with the 106 kDa polypeptide which specifically binds to poly(A) in an ATP-labile linkage.

Column liquid chromatography of integral membrane proteins

Biological membranes have as a major function the compartmentation of biological processes in cells and organelles. They consist of a bilayer of phospholipid molecules in which proteins are embedded. These integral membrane proteins, which cross the bilayer once or several times, generally have a higher than average hydrophobicity and tend to aggregate. Detergents are needed to remove integral membrane proteins from the lipid bilayer and they have to be present during further chromatographic purification. Predominantly, four modes of HPLC have been used alone or in combination for the purification of integral membrane proteins. These are based on differences of proteins in size (size-exclusion chromatography, SEC), electrostatic interaction (ion-exchange chromatography, IEC), bioaffinity (bioaffinity chromatography, BAC) and hydrophobic interaction (reversed-phase chromatography, RPC, and hydrophobic-interaction chromatography, HIC). SEC, IEC, BAC and HIC are used under relatively mild conditions, and buffer systems generally contain a non-ionic detergent. RPC generally has a denaturing effect on the protein and should preferably be used for the purification of integral membrane proteins smaller than 50 kD.

Hydrophobic interactions of brush border alkaline phosphatases: the role of phosphatidyl inositol

Tissue-specific (intestinal) and tissue-nonspecific (kidney) rat alkaline phosphatases are released from their respective brush border membranes by different enzymes. To elucidate the mechanism underlying their membrane attachment, we tested the ability of these enzymes to partition into lipid or aqueous phases both before and after treatment with phospholipases and proteases. Interaction with Triton X-114 micelles was eliminated or decreased by treatment of intestinal enzyme with phospholipase A2 or papain, while only phosphatidylinositol (PI)-specific phospholipase C (PIPLC) and subtilisin were effective with the kidney enzyme. Binding to octyl Sepharose for the intestinal enzyme was decreased by phospholipase A2 more than by PIPLC, whereas the reverse was true for the kidney enzyme. Treatment with phospholipases decreased the apparent mass of the phosphatases by 50-80 kDa, presumably due to loss of bound lipid and detergent. PIPLC treatment of the kidney, but not the intestinal enzyme, prevented binding of the phosphatase to phospholipid vesicles. These results show that both enzymes are bound to respective membranes by hydrophobic anchor peptides to which phospholipids are bound. However, their sensitivity to phospholipases is different. The data are consistent with the hypothesis that, in the kidney enzyme, the PI is bound covalently, while with the intestinal enzyme, binding of PI appears to be tight but not covalent.