GP-2/THP gene family encodes self-binding glycosylphosphatidylinositol-anchored proteins in apical secretory compartments of pancreas and kidney.

Abstract

A family of homologous genes is shown to encode GP-2, the major glycosylphosphatidylinositol (GPI)-linked glycoprotein of pancreatic zymogen granule membranes, and Tamm-Horsfall protein (THP), a GPI-linked glycoprotein associated with apical vesicles in kidney thick ascending limb of Henle (TALH) cells. The C-terminal regions of GP-2 (Asp5s-Phe-5-) and THP (Asp'75-IHis'4") from rat show 53% identity, 86% similarity, and 26 conserved cysteine residues including one epidermal growth factor motif. The unique N-terminal domain of rat THP (unique-THP, shows four conserved epidermal growth factor motifs, three in tandem and one in reverse orientation. GP-2 homologues are observed in a wide variety of epithelial cells, several of which contain highly regulated secretory processes. GP-2 released from zymogen granule membranes with phosphatidylinositol phospholipase C reacts with anti-cross-reactive determinant antibody (anti-CRD), confirming the GPI nature of the pancreatic homologue. In contrast, GP-2 and THP, released endogenously from pancreas and kidney, respectively, do not react with anti-cross-reactive determinant antibody, suggesting alternative enzymatic mechanisms for their physiological release. Globular domains of GP-2 and THP, but not albumin,show pH-and ion-dependent self-association in vitro. The GP-2/THP family appears to represent a newly discovered class of GPI-anchored proteins, which may utilize pH-and ion-dependent self-association mechanisms for establishing membrane (micro)domains targeted to intracellular secretory compartments.A growing number of hydrophilic proteins have recently been shown to be linked to membranes via a glycosylphosphatidylinositol (GPI) anchor (1, 2). Demonstrating diverse activities in mammalian cells, including cell adhesion, T-cell activation, and hydrolytic activities, GPI-anchored proteins are normally found attached to the outer surface of the plasma membrane. In polarized Madin-Darby canine kidney cells in culture, endogenous GPI-anchored and phosphatidylinositol phospholipase C (PI-PLC)-releasable proteins appeared to be restricted to the apical plasma membrane (3). In some cell lines, the surface expression and release of GPIlinked proteins is modulated by serum starvation or insulin (4). However, it is not clear why these proteins are tethered to membranes by GPI-lipid anchors or what role might be served by their potential release from the membrane (1). We have recently cloned and characterized rat (5) and dog GP-2 (6), the major glycoprotein of zymogen granule membranes (ZGMs) in the exocrine pancreas, and demonstrated that the membrane form of GP-2, targeted to apical plasma membranes via regulated exocytosis, is attached to the ectoleaflet of granule membranes by a GPI anchor. After granule assembly, we have also shown that GP-2 is released to the granule content fraction by a pH-dependent anchorcleavage activity associated with granule membranes (6). However, the function of GP-2 in ZGMs has remained obscure and its release from granule membranes into...

A Cluster of Mutations in the UMOD Gene Causes Familial Juvenile Hyperuricemic Nephropathy with Abnormal Expression of Uromodulin

“…Although uromodulin is the most abundant protein in the normal urine (33), its biologic role remains enigmatic. Uromodulin is a glycosylphosphatidylinositol (GPI) anchor-linked protein that is located to the apical membrane of tubular cells lining the TAL and DCT (34). The roles of uromodulin in the kidney may include modulation of cell adhesion (35) and signal transduction by interaction with protein kinases (36) and, more specific, inhibition of calcium oxalate crystal aggregation, formation of urinary casts, defense against urinary tract infection, and modulation of urine concentrating ability (28).…”

“…It is interesting that no mutation is predicted to result in premature termination of translation. The clustering of mutations in exon 4 is likely to be significant considering (1) a strong sequence conservation in evolution (38); (2) an approximately 53% sequence similarity with glycoprotein-2 (GP-2), a zymogen granule GPI-linked protein that, like uromodulin, is released from apical membranes to form large aggregates in solution (34,39); (3) the high number of cysteine residues in this part of the protein (24); and (4) the presence of three epidermal growth factor-like (EGF) repeats, able to interact with structurally related ligands (24,25,40) (Figure 1B). EGF domains represent one of the most commonly identified protein modules that mediate protein-protein interactions (40).…”

Crohn’s disease specific pancreatic antibodies: clinical and pathophysiological challenges

“…However, much to the surprise of gastroenterologists, this hypothesis could not be supported by the pathophysological features of a GP2-deficient mouse model [63]. Therefore, research on GP2's urinary homologue, the Tamm-Horsfall protein (THP) or uromodulin, has also been considered important to reveal GP2's physiological functions [64,65]. THP is the most abundant urinary protein which is secreted by renal tubular epithelial cells of the ascending limb of the loop of Henle in the urinary tract [65,66].…”

“…Therefore, research on GP2's urinary homologue, the Tamm-Horsfall protein (THP) or uromodulin, has also been considered important to reveal GP2's physiological functions [64,65]. THP is the most abundant urinary protein which is secreted by renal tubular epithelial cells of the ascending limb of the loop of Henle in the urinary tract [65,66]. Interestingly, both glycoproteins share one common ancestor gene which evolved separately during the phylogenesis to acquire tissue specificity in the digestive and urinary systems [67].…”

Localization of N‐linked carbohydrate chains in glycoprotein ZPA of the bovine egg zona pellucida

“…The localization of the spermligand active high-mannose-type chain in bovine ZPB and ZPC should be clarified. Besides the zona proteins, the ZP domain has been found in several proteins: TGF-b type III receptor (betaglycan) from fetal and adult tissues [28,29], uromodulin (urinary Tamm-Horsfall glycoprotein of pregnant woman) [30], the major zymogen granule membrane protein (GP-2) [31], and the proteins in the uterus and oviduct [32,33]. Furthermore, tectorins in the tectorial membranes responsible for hearing also have a ZP domain [34,35].…”