The gastric H,K-ATPase, a member of the P 2 -type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an α,β-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic α subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The β subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the β subunit contains six or seven Nlinked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E 1 (ion site in) to an E 2 (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E 2 P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E 2 P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E 1 configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

ized distribution of plasma membrane transporters and receptors in epithelia is essential for vectorial functions of epithelia. This polarity is maintained by sorting of membrane proteins into apical or basolateral transport containers in the transGolgi network and/or endosomes followed by their delivery to the appropriate plasma membrane domains. Sorting depends on the recognition of sorting signals in proteins by specific sorting machinery. In the present review, we summarize experimental evidence for and against the hypothesis that N-glycans attached to the membrane proteins can act as apical sorting signals. Furthermore, we discuss the roles of N-glycans in the apical sorting event per se and their contribution to folding and quality control of glycoproteins in the endoplasmic reticulum or retention of glycoproteins in the plasma membrane. Finally, we review existing hypotheses on the mechanism of apical sorting and discuss the potential roles of the lectins, VIP36 and galectin-3, as putative apical sorting receptors. apical sorting; apical membrane retention; H-K-ATPase ␤-subunit; lectin EPITHELIAL CELLS CARRY OUT vectorial transport that requires polarized distribution of transporters and receptors to apical or basolateral membrane domains. These domains are separated by tight junctions that connect neighboring cells in the epithelial monolayer and act as diffusion barriers to prevent mixing of apical and basolateral membrane components (22). Asymmetric distribution of plasma membrane proteins is accomplished by their sorting into apical and basolateral containers in the trans-Golgi network (TGN) and/or endosomes followed by vectorial transport of these containers and insertion and retention of the proteins in the appropriate plasma membrane domains. Sorting depends on recognition of apical and basolateral sorting signals within the proteins by cellular sorting machinery (20,58,59,75,76).Numerous studies have indicated that both O-and N-glycans attached to the extracellular domains of some membrane proteins are important for apical location of these proteins. This review will focus on the role of N-glycans in polarized distribution of plasma membrane proteins in epithelia. The role of O-glycans in apical sorting has been described in several recent excellent reviews (18, 71) and will not be discussed further here.A putative role of N-glycans as apical sorting signals was postulated more than 10 years ago (24, 31). However, this hypothesis remains controversial primarily because N-glycans are important for the processes that precede or follow the actual sorting event, such as protein folding, quality control, endoplasmic reticulum (ER)-associated degradation, ER-to-Golgi trafficking, and retention of glycoproteins in the apical membrane. Therefore, merely examining the effect of altering the number or the nature of N-linked glycans on the relative abundance of the glycoprotein in the apical membrane, as has been done in many of the studies reported, does not allow one to distinguish between effects on apica...

Background & Aims The pathogenic mechanism of pancreatitis is poorly understood. Recent evidence implicates defective autophagy in pancreatitis responses; however, the pathways mediating impaired autophagy in pancreas remain largely unknown. Here, we investigate the role of lysosome associated membrane proteins (LAMPs) in pancreatitis. Methods We analyzed changes in LAMPs in experimental models and human pancreatitis, and the underlying mechanisms: LAMP de-glycosylation and degradation. LAMP cleavage by cathepsin B (CatB) was analyzed by mass spectrometry. We used mice deficient in LAMP-2 to assess its role in pancreatitis. Results Pancreatic levels of LAMP-1 and LAMP-2 greatly decrease across various pancreatitis models and in human disease. Pancreatitis does not trigger LAMPs’ bulk de-glycosylation, but induces their degradation via CatB-mediated cleavage of LAMP molecule close to the boundary between luminal and transmembrane domains. LAMP-2 null mice spontaneously develop pancreatitis that begins with acinar cell vacuolization due to impaired autophagic flux, and progresses to severe pancreas damage characterized by trypsinogen activation, macrophage-driven inflammation, and acinar cell death. LAMP-2 deficiency causes a decrease in pancreatic digestive enzymes content, stimulates the basal and inhibits CCK-induced amylase secretion by acinar cells. The effects of LAMP-2 knockout and acute cerulein pancreatitis overlap, which corroborates the pathogenic role of LAMP decrease in experimental pancreatitis models. Conclusions The results indicate a critical role for LAMPs, particularly LAMP-2, in maintaining pancreatic acinar cell homeostasis, and provide evidence that defective lysosomal function, resulting in impaired autophagy, leads to pancreatitis. Mice with LAMP-2 deficiency present a novel genetic model of human pancreatitis caused by lysosomal/autophagic dysfunction.