Identification of the microvillar 110-kDa calmodulin complex (myosin-1) in kidney.

The epithelial layer lining the proximal convoluted tubule of mammalian kidney contains a brush border of numerous microvilli. These microvilli appear in structure to be very similar to the microvilli on epithelial cells of the small intestine. Microvilli found in both the small intestine and the proximal convoluted tubules in kidney have a core bundle of actin filaments bundled by the accessory proteins villin and fimbrin. Along the length of intestinal microvilli, lateral links can be observed to connect the core bundle of actin filaments to the membrane. These cross-bridges are comprised of a 110-kDa calmodulin complex which belongs to a class of single-headed myosin molecules, collectively referred to as myosin-1. We now report that an analogous calmodulin-binding polypeptide of 105 kDa has been identified in rat kidney cortex. The 105-kDa polypeptide is preferentially found in purified kidney brush borders, can be extracted with ATP, and co-elutes with calmodulin on gel filtration and anion exchange chromatography. Fractions containing the 105-kDa polypeptide exhibit a modest ATPase activity in buffer containing CaCl2. The partially purified 105-kDa polypeptide will bind iodinated calmodulin and will sediment with F-actin in buffer containing ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or Ca2+. The addition of ATP partially reverses this association with F-actin. These results indicate that myosin-1, in addition to its presence in intestinal brush borders, is present in the brush border of kidney. We also provide preliminary evidence to indicate that the 105-kDa polypeptide is not restricted to tissues possessing a brush border.     

Tyrosine sulfation, a post-translational modification of microvillar enzymes in the small intestinal enterocyte.

Protein sulfation in small intestinal epithelial cells was studied by labelling of organ cultured mucosal explants with [35S]-sulfate. Six bands in SDS-PAGE became selectively labelled; four, of 250, 200, 166 and 130 kd, were membrane-bound and two, of 75 and 60 kd, were soluble. The sulfated membrane-bound components were all enriched in the microvillar fraction but either absent or barely detectable in intracellular or basolateral membranes. Immunopurification of sucrase-isomaltase, maltase-glucoamylase, aminopeptidase N and aminopeptidase A showed that these microvillar enzymes become sulfated. Most if not all the sulfate was bound to tyrosine residues rather than to the carbohydrate of the microvillar enzymes, showing that this type of modification can occur on plasma membrane proteins as well as on secretory proteins.     

Domains of increased thickness in microvillar membranes of the small intestinal enterocyte

Abstract

The apical surface of the enterocyte is sculpted into a dense array of cylindrical microvillar protrusions by supporting actin filaments. Membrane microdomains (rafts) enriched in cholesterol and glycosphingolipids comprise roughly 50% of the microvillar membrane and play a vital role in orchestrating absorptive/digestive action of dietary nutrients at this important cellular interface. Increased membrane thickness is believed to be a morphological characteristic of rafts. Thus, we investigated whether the high contents of lipid rafts in the microvillar membrane is reflected in local variations in membrane thickness. We measured membrane thickness directly from electron micrographs of sections of fixed mucosal tissue. Indeed, mapping of the microvillar membrane revealed a biphasic distribution of membrane thickness. As a point of reference the thickness distribution of the basolateral membrane was clearly monophasic. The encountered domains of increased thickness (DITs) occupied 48% of the microvillar membrane and from the data we estimated the area of a single DIT to have a lower limit of 600 nm². In other experiments we mapped the organization of biochemically defined lipid rafts by immunogold labeling of alkaline phosphatase, a well documented raft marker. Strikingly, the alkaline phosphatase localized to distinct regions of the membrane in a pattern similar to the observed distribution of DITs. Although we were unable to measure membrane thickness directly on the immunogold labeled specimens, and thereby establish an unequivocal connection between DITs and rafts, we conclude that the brush border membrane of the enterocyte contains microdomains distinguishable both by membrane morphology and protein composition.     

Pig intestinal microvillar maltase-glucoamylase. Structure and membrane insertion

The NH2-terminal sequence (25 residues) of amphiphilic single polypeptide chain maltase-glucoamylase (EC 3.2.1.20) was determined by gas-phase sequencing. The result indicates that the NH2-terminal segment anchors the enzyme to the microvillar membrane. The single-chain form and the proteolytically processed two-chain form have two distinct active sites differing in heat stability. However, both sites are sensitive to chonduritol B-epoxide and have similar substrate specificity. The amphiphilic single-chain maltase-glucoamylase and the amphiphilic proteolytically processed form were inserted into liposomes and studied by electron microscopy. The results showed that the enzyme is predominantly present as a homodimeric complex in the membrane.     

Antigenic secreted proteins from Haemophilus paragallinarum. A 110-kDa putative RTX protein

Haemophilus paragallinarum is the causal agent of infectious coryza, an economically important disease for the poultry industry. This bacterium secreted proteins of 25-110 kDa during its growth in brain heart infusion, tryptic soy broth, or Luria-Bertani glucose phosphate media, all lacking serum. Some of these proteins were recognized by sera from chickens experimentally infected with H. paragallinarum. A 110-kDa protein was recognized by a serum pool from convalescent-phase pigs naturally infected with Actinobacillus pleuropneumoniae, and also by a rabbit polyclonal serum against Apx I as well as a rabbit serum against Mannheimia haemolytica leukotoxin, suggesting the presence of an RTX-like protein in H. paragallinarum. H. paragallinarum secreted proteins could be important immunogens in the control of infectious coryza.     

Membrane insertion of uracil permease, a polytopic yeast plasma membrane protein.

Uracil permease is a multispanning protein of the Saccharomyces cerevisiae plasma membrane which is encoded by the FUR4 gene and produced in limited amounts. It has a long N-terminal hydrophilic segment, which is followed by 10 to 12 putative transmembrane segments, and a hydrophilic C terminus. The protein carries seven potential N-linked glycosylation sites, three of which are in its N-terminal segment. Overexpression of this permease and specific antibodies were used to show that uracil permease undergoes neither N-linked glycosylation nor proteolytic processing. Uracil permease N-terminal segments of increasing lengths were fused to a reporter glycoprotein, acid phosphatase. The in vitro and in vivo fates of the resulting hybrid proteins were analyzed to identify the first signal anchor sequence of the permease and demonstrate the cytosolic orientation of its N-terminal hydrophilic sequence. In vivo insertion of the hybrid protein bearing the first signal anchor sequence of uracil permease into the endoplasmic reticulum membrane was severely blocked in sec61 and sec62 translocation mutants.     

Membrane protein insertion: mixing eukaryotic and prokaryotic concepts

Proteins are translocated across or inserted into membranes by machines that are composed of soluble and membrane-anchored subunits. The molecular action of these machines and their evolutionary origin are at present the focus of intense research. For instance, our understanding of the mode of insertion of beta-barrel membrane proteins into the outer membrane of endosymbiotically derived organelles has increased rapidly during the past few years. In particular, the identification of the Omp85/YaeT-involving pathways in Neisseria meningitidis, Escherichia coli and cyanobacteria, and homologues of Omp85/YaeT in chloroplasts and mitochondria, has provided new clues about the ancestral beta-barrel protein insertion pathway. This review focuses on recent advances in the elucidation of the evolutionarily conserved concepts that underlie the translocation and insertion of beta-barrel membrane proteins.     

Unique peptide substrate binding properties of 110-kDa heat-shock protein (Hsp110) determine its distinct chaperone activity

The molecular chaperone 70-kDa heat-shock proteins (Hsp70s) play essential roles in maintaining protein homeostasis. Hsp110, an Hsp70 homolog, is highly efficient in preventing protein aggregation but lacks the hallmark folding activity seen in Hsp70s. To understand the mechanistic differences between these two chaperones, we first characterized the distinct peptide substrate binding properties of Hsp110s. In contrast to Hsp70s, Hsp110s prefer aromatic residues in their substrates, and the substrate binding and release exhibit remarkably fast kinetics. Sequence and structure comparison revealed significant differences in the two peptide-binding loops: the length and properties are switched. When we swapped these two loops in an Hsp70, the peptide binding properties of this mutant Hsp70 were converted to Hsp110-like, and more impressively, it functionally behaved like an Hsp110. Thus, the peptide substrate binding properties implemented in the peptide-binding loops may determine the chaperone activity differences between Hsp70s and Hsp110s.     

Enhanced phosphorylation of a nucleolar 110-kDa protein in rat liver by dietary manipulation

RNA polymerase 1 activity and nucleolar volume have been reported to increase in hepatocytes from rats fed a protein-free diet. Phosphorylation in vitro of a 110-kDa protein was enhanced in nuclei and nucleoli from livers of rats fed a protein-free diet. In nuclear extracts the 110-kDa protein in heat-treated nuclei was much more phosphorylated than from control liver. In contrast, casein kinase activity in the nuclear extract from control liver was comparable to that from livers of rats fed a protein-free diet. Nuclear extracts from control rat liver and livers of rats fed a protein-free diet were fractionated by DEAE-cellulose column chromatography. Casein kinase II (NII) eluted at around 0.17 M NaCl scarcely phosphorylates the 110-kDa protein. Chromatography of the nuclear extract from livers of rats fed a protein-free diet, but not from control liver, yielded fractions which eluted at 0.21-0.25 M NaCl and predominantly phosphorylated the 110-kDa protein. The phosphorylation of 110-kDa protein was not appreciably affected by a heparin concentration of 5 micrograms/ml, which completely inhibited casein kinase II. In addition, phosphorylation of the 110-kDa protein in liver nucleoli from rats fed a protein-free diet showed a lower sensitivity to heparin than that in control rat liver nucleoli. These results suggest that enhanced phosphorylation of the nuclear 110-kDa protein in livers from rats fed a protein-free diet is due to the induction of a 110-kDa protein kinase distinct from casein kinase II.     

Biphasic increase in the in vivo phosphorylation of nuclear 110-kDa protein during early lymphocyte transformation

A major 110-kDa phosphoprotein in rat liver and hepatoma (P 110) was identified in nuclear 0.2 M HCl extracts from rat lymph node cells. Stimulation with concanavalin A altered both the amount of P110 and, more strikingly, its in vivo phosphorylation in a typical biphasic manner with an initial maximum after 10-14 h. RNA synthesis showed a similar biphasic increase. Inhibition of hnRNA synthesis (5,6 dichlorobenzimidazoleriboside), but not of DNA synthesis (hydroxyurea), depressed both the amount of P110 and its phosphorylation markedly.     

Identification of a 110-kDa nonintegrin cell surface laminin-binding protein which recognizes an A chain neurite-promoting peptide.

Laminin is a potent promoter of neurite outgrowth, and a synthetic peptide of 19 amino acids, PA22-2, from the A chain has been found to promote process formation. Using peptide affinity chromatography, we have identified a 110-kDa, cell surface ligand from both neural cells and brain which binds this sequence. This binding protein does not share immunological identity with the B1 chain of integrin, and reduction does not alter its mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antibody to the 110-kDa protein stained cellular processes in vivo. Sequence analysis of the first 18 amino acids from the amino terminus yielded almost exact sequence identity with nucleolin, a major 110-kDa nucleolar phosphoprotein. Antibody to nucleolin, however, does not interact with the neural-derived, laminin-peptide-binding 110-kDa protein. The 110-kDa protein appears to be a ligand for a specific site on laminin.     

Membrane protein insertion into the endoplasmic reticulum - another channel tunnel?

The synthesis of biological membranes requires the insertion of proteins into a lipid bilayer. The rough endoplasmic reticulum of eukaryotic cells is a principal site of membrane biogenesis. The insertion of proteins into the membrane of the endoplasmic reticulum is mediated by a resident proteinaceous machinery. Over the last five years several different experimental approaches have provided information about the components of the machinery and how it may function.     

Membrane insertion and topology of the translocating chain-associating membrane protein (TRAM)

The translocating chain-associating membrane protein (TRAM) is a glycoprotein involved in the translocation of secreted proteins into the endoplasmic reticulum (ER) lumen and in the insertion of integral membrane proteins into the lipid bilayer. As a major step toward elucidating the structure of the functional ER translocation/insertion machinery, we have characterized the membrane integration mechanism and the transmembrane topology of TRAM using two approaches: photocross-linking and truncated C-terminal reporter tag fusions. Our data indicate that TRAM is recognized by the signal recognition particle and translocon components, and suggest a membrane topology with eight transmembrane segments, including several poorly hydrophobic segments. Furthermore, we studied the membrane insertion capacity of these poorly hydrophobic segments into the ER membrane by themselves. Finally, we confirmed the main features of the proposed membrane topology in mammalian cells expressing full-length TRAM.     

Biosynthesis of heparin. Purification of a 110-kDa mouse mastocytoma protein required for both glucosaminyl N-deacetylation and N-sulfation

The polymer modification process in the biosynthesis of heparin/heparan sulfate is initiated by N-deacetylation, followed by N-sulfation, of N-acetylglucosamine units. Chromatography of a detergent extract from mouse mastocytoma on wheat germ agglutinin-Sepharose yielded a protein fraction, eluted with 0.3 M N-acetylglucosamine, that expressed N-deacetylase activity, but only after recombination with proteins that did not bind to the lectin column. In subsequent purification of the active lectin-bound component, all assays were performed following addition of the unbound protein fraction. After two additional chromatography steps, on blue Sepharose and 3',5'-ADP-agarose, the lectin-binding N-deacetylase component had been purified about 4300-fold with an 11% yield and showed essentially a single band, corresponding to an apparent molecular weight of approximately 110,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of the purified 110-kDa protein showed that it contained, in addition to the N-deacetylase, N-sulfotransferase activity; however, the expression of N-sulfotransferase activity was independent of additional proteins. Backtracking the N-sulfotransferase through the purification scheme previously applied to the N-deacetylase showed the two enzyme activities to the N-deacetylase showed the two enzyme activities to be cofractionated in each separation step. It is proposed that the expression of glucosaminyl N-deacetylase activity depends on the concerted action of (at least) two protein components, one of which also possesses glucosaminyl N-sulfotransferase activity.     

The fission yeast dis3+ gene encodes a 110-kDa essential protein implicated in mitotic control.

The fission yeast mutant dis3-54 is defective in mitosis and fails in chromosome disjunction. Its phenotype is similar to that of dis2-11, a mutant with a mutation in the type 1 protein phosphatase gene. We cloned the dis3+ gene by transformation. Nucleotide sequencing predicts a coding region of 970 amino acids interrupted by a 164-bp intron at the 65th codon. The predicted dis3+ protein shares a weak but significant similarity with the budding yeast SSD1 or SRK1 gene product, the gene for which is a suppressor for the absence of a protein phosphatase SIT4 gene or the BCY1 regulatory subunit of cyclic AMP-dependent protein kinase. Anti-dis3 antibodies recognized the 110-kDa dis3+ gene product, which is part of a 250- to 350-kDa oligomer and is enriched in the nucleus. The cellular localization of the dis3+ protein is reminiscent of that of the dis2+ protein, but these two proteins do not form a complex. A type 1 protein phosphatase activity in the dis3-54 mutant extracts is apparently not affected. The dis3+ gene is essential for growth; gene disruptant cells do not germinate and fail in cell division. Increased dis3+ gene dosage reverses the Ts+ phenotype of a cdc25 wee1 strain, as does increased type 1 protein phosphatase gene dosage. Double mutant dis3 dis2 is lethal even at the permissive temperature, suggesting that the dis2+ and dis3+ genes may be functionally overlapped. The role of the dis3+ gene product in mitosis is unknown, but this gene product may be directly or indirectly involved in the regulation of mitosis.     

Membrane insertion kinetics of a protein domain in vivo. The bacterioopsin n terminus inserts co-translationally.

The pathway by which segments of a polytopic membrane protein are inserted into the membrane has not been resolved in vivo. We have developed an in vivo kinetic assay to examine the insertion pathway of the polytopic protein bacterioopsin, the apoprotein of Halobacterium salinarum bacteriorhodopsin. Strains were constructed that express the bacteriorhodopsin mutants I4C:H(6) and T5C:H(6), which carry a unique Cys in the N-terminal extracellular domain and a polyhistidine tag at the C terminus. Translocation of the N-terminal domain was detected using a membrane-impermeant gel shift reagent to derivatize the Cys residue of nascent radiolabeled molecules. Derivatization was assessed by gel electrophoresis of the fully elongated radiolabeled population. The time required to translocate and fully derivatize the Cys residues of I4C:H(6) and T5C:H(6) is 46 +/- 9 and 61 +/- 6 s, respectively. This is significantly shorter than the elongation times of the proteins, which are 114 +/- 26 and 169 +/- 16 s, respectively. These results establish that translocation of the bacterioopsin N terminus and insertion of the first transmembrane segment occur co-translationally and confirm the use of the assay to monitor the kinetics of polytopic membrane protein insertion in vivo.     

Membrane insertion of the Escherichia coli MalF protein in cells with impaired secretion machinery

The MalF protein is an integral membrane protein of Escherichia coli containing eight membrane-spanning stretches and a large periplasmic domain of approximately 180 amino acids. We have asked whether this protein is dependent for its membrane insertion on the bacterial secretion machinery specified by the sec genes. Using azide to inhibit the SecA protein and sec mutants to reduce the functioning of the machinery, we have studied the membrane assembly of MalF and beta-galactosidase and alkaline phosphatase fusions to MalF. In no case did we see an effect of reducing sec gene function on the insertion of MalF or fusion proteins. Selection for mutants that would cause internalization of a MalF-beta-galactosidase hybrid protein yielded no mutations in sec genes. Our results suggest that MalF can assemble in the membrane independently of the bacterial secretion machinery.     

Membrane insertion stabilizes the structure of TrwB, the R388 conjugative plasmid coupling protein

TrwB is an integral membrane protein that plays a crucial role in the conjugative process of plasmid R388. We have recently shown [Vecino et al., Biochim. Biophys. Acta 1798(11), 2160-2169 (2010)] that TrwB can be reconstituted into liposomes, and that bilayer incorporation increases its affinity for nucleotides and its specificity for ATP. In the present contribution we examine the structural effects of membrane insertion on TrwB, by comparing the protein in reconstituted form and in the form of protein/lipid/detergent mixed micelles. TrwB was reconstituted in PE:PG:CL (76.3:19.6:4.1mol ratio) with a final 99:1 lipid:protein mol ratio. This lipid mixture is intended to mimic the bacterial inner membrane composition, and allows a more efficient reconstitution than other lipid mixtures tested. The studies have been carried out mainly using infrared spectroscopy, because this technique provides simultaneously information on both the lipid and protein membrane components. Membrane reconstitution of TrwB is accompanied by a decrease in β-sheet contents and an increase in β-strand structures, probably related to protein-protein contacts in the bilayer. The predominant α-helical component remains unchanged. The bilayer-embedded protein becomes thermally more stable, and also more resistant to trypsin digestion. The properties of the bilayer lipids are also modified in the presence of TrwB, the phospholipid acyl chains are slightly ordered, and the phosphate groups at the interface become more accessible to water. In addition, we observe that the protein thermal denaturation affects the lipid thermal transition profile.