Characteristics and structural requirements of apical sorting of the rat growth hormone through the O-glycosylated stalk region of intestinal sucrase-isomaltase

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Intestinal peptidases form functional complexes with the neutral amino acid transporter B(0)AT1

The brush-border membrane of the small intestine and kidney proximal tubule are the major sites for the absorption and re-absorption of nutrients in the body respectively. Transport of amino acids is mediated through the action of numerous secondary active transporters. In the mouse, neutral amino acids are transported by B(0)AT1 [broad neutral ((0)) amino acid transporter 1; SLC6A19 (solute carrier family 6 member 19)] in the intestine and by B(0)AT1 and B(0)AT3 (SLC6A18) in the kidney. Immunoprecipitation and Blue native electrophoresis of intestinal brush-border membrane proteins revealed that B(0)AT1 forms complexes with two peptidases, APN (aminopeptidase N/CD13) and ACE2 (angiotensin-converting enzyme 2). Physiological characterization of B(0)AT1 expressed together with these peptidases in Xenopus laevis oocytes revealed that APN increased the substrate affinity of the transporter up to 2.5-fold and also increased its surface expression (V(max)). Peptide competition experiments, in silico modelling and site-directed mutagenesis of APN suggest that the catalytic site of the peptidase is involved in the observed changes of B(0)AT1 apparent substrate affinity, possibly by increasing the local substrate concentration. These results provide evidence for the existence of B(0)AT1-containing digestive complexes in the brush-border membrane, interacting differentially with various peptidases, and responding to the dynamic needs of nutrient absorption in the intestine and kidney.

Characterization of mouse sperm TMEM190, a small transmembrane protein with the trefoil domain: evidence for co-localization with IZUMO1 and complex formation with other sperm proteins

TMEM190, a small transmembrane protein containing the trefoil domain, was previously identified by our proteomic analysis of mouse sperm. Two structural features of TMEM190, ‘trefoil domain’ and ‘small transmembrane protein’, led us to hypothesize that this protein forms a protein–protein complex required during fertilization, and we characterized TMEM190 by biochemical, cytological, and genetic approaches. We showed in this study that the mouse Tmem190 gene exhibits testis-specific mRNA expression and that the encoded RNA is translated into a 19-kDa protein found in both testicular germ cells and cauda epididymal sperm. Treatment of the cell surface with proteinase K, subcellular fractionation, and immunofluorescence assay all revealed that mouse TMEM190 is an inner-acrosomal membrane protein of cauda epididymal sperm. During the acrosome reaction, TMEM190 partly relocated onto the surface of the equatorial segment, on which sperm–oocyte fusion occurs. Moreover, TMEM190 and IZUMO1, which is an immunoglobulin-like protein required for gamete fusion, co-localized in mouse sperm both before and after the acrosome reaction. However, immunoprecipitates of TMEM190 contained several sperm proteins, but did not include IZUMO1. These findings suggest that a mouse sperm protein complex(es) including TMEM190 plays an indirect role(s) in sperm–oocyte fusion. The role(s), if any, is probably dispensable since Tmem190-null male mice were normally fertile.

Rescue of Drosophila Melanogaster l(2)35Aa lethality is only mediated by polypeptide GalNAc-transferase pgant35A, but not by the evolutionary conserved human ortholog GalNAc-transferase-T11

The Drosophila l(2)35Aa gene encodes a UDP-N-acetylgalactosamine: Polypeptide N-acetylgalactosaminyltransferase, essential for embryogenesis and development (J. Biol. Chem. 277, 22623-22638; J. Biol. Chem. 277, 22616-22). l(2)35Aa, also known as pgant35A, is a member of a large evolutionarily conserved family of genes encoding polypeptide GalNAc-transferases. Phylogenetic and functional analyses have proposed that subfamilies of orthologous GalNAc-transferase genes are conserved in species, suggesting that they serve distinct functions in vivo. Based on sequence alignments, pgant35A and human GALNT11 are thought to belong to a distinct subfamily. Recent in vitro studies have shown that pgant35A and pgant7, encoding enzymes from different subfamilies, prefer different acceptor substrates, whereas the orthologous pgant35A and human GALNT11 gene products possess, 1) conserved substrate preferences and 2) similar acceptor site preferences in vitro. In line with the in vitro pgant7 studies, we show that l(2)35Aa lethality is not rescued by ectopic pgant7 expression. Remarkably and in contrast to this observation, the human pgant35A ortholog, GALNT11, was shown not to support rescue of the l(2)35Aa lethality. By use of genetic "domain swapping" experiments we demonstrate, that lack of rescue was not caused by inappropriate sub-cellular targeting of functionally active GalNAc-T11. Collectively our results show, that fly embryogenesis specifically requires functional pgant35A, and that the presence of this gene product during fly embryogenesis is functionally distinct from other Drosophila GalNAc-transferase isoforms and from the proposed human ortholog GALNT11.

Apical trafficking in epithelial cells: signals, clusters and motors

In the early days of epithelial cell biology, researchers working with kidney and/or intestinal epithelial cell lines and with hepatocytes described the biosynthetic and recycling routes followed by apical and basolateral plasma membrane (PM) proteins. They identified the trans-Golgi network and recycling endosomes as the compartments that carried out apical-basolateral sorting. They described complex apical sorting signals that promoted association with lipid rafts, and simpler basolateral sorting signals resembling clathrin-coated-pit endocytic motifs. They also noticed that different epithelial cell types routed their apical PM proteins very differently, using either a vectorial (direct) route or a transcytotic (indirect) route. Although these original observations have generally held up, recent studies have revealed interesting complexities in the routes taken by apically destined proteins and have extended our understanding of the machinery required to sustain these elaborate sorting pathways. Here, we critically review the current status of apical trafficking mechanisms and discuss a model in which clustering is required to recruit apical trafficking machineries. Uncovering the mechanisms responsible for polarized trafficking and their epithelial-specific variations will help understand how epithelial functional diversity is generated and the pathogenesis of many human diseases.

Identification of a novel apical sorting motif and mechanism of targeting of the M2 muscarinic acetylcholine receptor

Previous studies have shown that the M2 receptor is localized at steady state to the apical domain in Madin-Darby canine kidney (MDCK) epithelial cells. In this study, we identify the molecular determinants governing the localization and the route of apical delivery of the M2 receptor. First, by confocal analysis of a transiently transfected glycosylation mutant in which the three putative glycosylation sites were mutated, we determined that N-glycans are not necessary for the apical targeting of the M2 receptor. Next, using a chimeric receptor strategy, we found that two independent sequences within the M2 third intracellular loop can confer apical targeting to the basolaterally targeted M4 receptor, Val270-Lys280 and Lys280-Ser350. Experiments using Triton X-100 extraction followed by OptiPrep density gradient centrifugation and cholera toxin beta-subunit-induced patching demonstrate that apical targeting is not because of association with lipid rafts. 35S-Metabolic labeling experiments with domain-specific surface biotinylation as well as immunocytochemical analysis of the time course of surface appearance of newly transfected confluent MDCK cells expressing FLAG-M2-GFP demonstrate that the M2 receptor achieves its apical localization after first appearing on the basolateral domain. Domain-specific application of tannic acid of newly transfected cells indicates that initial basolateral plasma membrane expression is required for subsequent apical localization. This is the first demonstration that a G-protein-coupled receptor achieves its apical localization in MDCK cells via transcytosis.

Polypeptide GalNAc-transferase T3 and familial tumoral calcinosis. Secretion of fibroblast growth factor 23 requires O-glycosylation

Mutations in the gene encoding the glycosyltransferase polypeptide GalNAc-T3, which is involved in initiation of O-glycosylation, were recently identified as a cause of the rare autosomal recessive metabolic disorder familial tumoral calcinosis (OMIM 211900). Familial tumoral calcinosis is associated with hyperphosphatemia and massive ectopic calcifications. Here, we demonstrate that the secretion of the phosphaturic factor fibroblast growth factor 23 (FGF23) requires O-glycosylation, and that GalNAc-T3 selectively directs O-glycosylation in a subtilisin-like proprotein convertase recognition sequence motif, which blocks processing of FGF23. The study suggests a novel posttranslational regulatory model of FGF23 involving competing O-glycosylation and protease processing to produce intact FGF23.

Disaccharide digestion: clinical and molecular aspects

Sugars normally are absorbed in the small intestine. When carbohydrates are malabsorbed, the osmotic load produced by the high amount of low molecular weight sugars and partially digested starches in the small intestine can cause symptoms of intestinal distention, rapid peristalsis, and diarrhea. Colonic bacteria normally metabolize proximally malabsorbed dietary carbohydrate through fermentation to small fatty acids and gases (ie, hydrogen, methane, and carbon dioxide). When present in large amounts, the malabsorbed sugars and starches can be excreted in the stool. Sugar intolerance is the presence of abdominal symptoms related to the proximal or distal malabsorption of dietary carbohydrates. The symptoms consist of meal-related abdominal cramps and distention, increased flatulence, borborygmus, and diarrhea. Infants and young children with carbohydrate malabsorption show more intense symptoms than adults; the passage of undigested carbohydrates through the colon is more rapid and is associated with detectable carbohydrates in copious watery acid stools. Dehydration often follows feeding of the offending sugar. In this review we present the clinical and current molecular aspects of disaccharidase digestion.

Apical Transport and Folding of Prostate-specific Membrane Antigen Occurs Independent of Glycan Processing

Prostate-specific membrane antigen (PSMA) is an integral cell-surface membrane glycoprotein that is overexpressed in prostate carcinomas rendering it an appropriate target for antibody-based therapeutic strategies. The biosynthesis of PSMA in transfected COS-1 cells reveals a slow conversion of mannose-rich to complex glycosylated PSMA compatible with slow transport kinetics from the endoplasmic reticulum to the Golgi. Importantly, mannose-rich PSMA persists as a trypsin-sensitive protein throughout its entire life cycle, and only Golgi-located PSMA glycoforms acquire trypsin resistance. This resistance, used here as a tool to examine correct folding, does not depend on the type of glycosylation, because different PSMA glycoforms generated in the presence of inhibitors of carbohydrate processing in the Golgi are also trypsin resistant. The conformational transition of PSMA to a correctly folded molecule is likely to occur in the Golgi and does not implicate ER molecular chaperones, such as BiP. We show here that PSMA is not only heavily N-but also O-glycosylated. The question arising is whether glycans, which do not play a role in folding of PSMA, are implicated in its transport to the cell surface. Neither the cell-surface expression of PSMA nor its efficient apical sorting in polarized Madin-Darby canine kidney cells are influenced by modulators of N- and O-glycosylation. The acquisition of folding determinants in the Golgi, therefore, is an essential prerequisite for protein trafficking and sorting of PSMA and suggests that altered or aberrant glycosylation often occurring during tumorigenesis has no regulatory effect on the cell-surface expression of PSMA.

Role of N- and O-glycans in polarized biosynthetic sorting

The maintenance of proper epithelial function requires efficient sorting of newly synthesized and recycling proteins to the apical and basolateral surfaces of differentiated cells. Whereas basolateral protein sorting signals are generally confined to their cytoplasmic regions, apical targeting signals have been identified that localize to luminal, transmembrane, and cytoplasmic aspects of proteins. In the past few years, both N- and O-linked glycans have been identified as apical sorting determinants. Glycan structures are extraordinarily diverse and have tremendous information potential. Moreover, because the oligosaccharides added to a given protein can change depending on cell type and developmental stage, the potential exists for altering sorting pathways by modulation of the expression pattern of enzymes involved in glycan synthesis. In this review, we discuss the evidence for glycan-mediated apical sorting along the biosynthetic pathway and present possible mechanisms by which these common and heterogeneous posttranslational modifications might function as specific sorting signals.

A novel type of detergent-resistant membranes may contribute to an early protein sorting event in epithelial cells

One sorting mechanism of apical and basolateral proteins in epithelial cells is based on their solubility profiles with Triton X-100. Nevertheless, apical proteins themselves are also segregated beyond the trans-Golgi network by virtue of their association or nonassociation with cholesterol/sphingolipid-rich microdomains (Jacob, R., and Naim, H. Y. (2001) Curr. Biol. 11, 1444-1450). Therefore, extractability with Triton X-100 does not constitute an absolute criterion of protein sorting. Here, we investigate the solubility patterns of apical and basolateral proteins with other detergents and demonstrate that the mild detergent Tween 20 is adequate to discriminate between apical and basolateral proteins during early stages in their biosynthesis. Although the mannose-rich forms of the apical proteins sucrase-isomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV reveal similar solubility profiles comprising soluble and nonsoluble fractions, the basolateral proteins, vesicular stomatitis virus G protein, major histocompatibility complex class I, and CD46 are entirely soluble with this detergent. The insoluble Tween 20 membranes are enriched in phosphatidylinositol and phosphatidylglycerol compatible with their synthesis in the endoplasmic reticulum and the existence of a novel class of detergent-resistant membranes. The association of the mannose-rich biosynthetic forms of the apical proteins, sucraseisomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV with the Tween 20-resistant membranes suggests an early polarized sorting mechanism prior to maturation in the Golgi apparatus.

Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development.

O-glycosylation as a sorting determinant for cell surface delivery in yeast

Little is known about the mechanisms that determine localization of proteins to the plasma membrane in Saccharomyces cerevisiae. The length of the transmembrane domains and association of proteins with lipid rafts have been proposed to play a role in sorting to the cell surface. Here, we report that Fus1p, an O-glycosylated integral membrane protein involved in cell fusion during yeast mating, requires O-glycosylation for cell surface delivery. In cells lacking PMT4, encoding a mannosyltransferase involved in the initial step of O-glycosylation, Fus1p was not glycosylated and accumulated in late Golgi structures. A chimeric protein lacking O-glycosylation motif was missorted to the vacuole and accumulated in late Golgi in wild-type cells. Exocytosis of this protein could be restored by addition of a 33-amino acid portion of an O-glycosylated sequence from Fus1p. Our data suggest that O-glycosylation functions as a sorting determinant for cell surface delivery of Fus1p.

Involvement of glycosylation in the intracellular trafficking of glycoproteins in polarized epithelial cells

The surface of epithelial cells is composed of apical and basolateral domains with distinct structure and function. This polarity is maintained by specific sorting mechanisms occurring in the Trans-Golgi Network. Peptidic signals are responsible for the trafficking via clathrin-coated vesicles by means of an interaction with an adaptor complex (AP). The basolateral targeting is mediated by AP-1B, which is specifically expressed in epithelial cells. In contrast, the apical targeting is proposed to occur via apical raft carriers. It is thought that apically targeted glycoproteins contain glycan signals that would be responsible for their association with rafts and for apical targeting. However, the difficulty in terms of acting specifically on a single step of glycosylation did not allow one to identify such a specific signal. The complete inhibition of the processing of N-glycans by tunicamycin often results in an intracellular accumulation of unfolded proteins in the Golgi. Similarly, inhibition of O-glycosylation can be obtained by competitive substrates which gave a complex pattern of inhibition. Therefore, it is still unknown if glycosylation acts in an indirect manner, i.e. by modifying the folding of the protein, or in a specific manner, such as an association with specific lectins.