Structural hierarchy of regulatory elements in the folding and transport of an intestinal multidomain protein

Analysis of Amyloid Fibrillation of Two Family 1 Glycoside Hydrolases

The formation and analysis of amyloid fibers by two β-glucosidases, BglA and BglB, belonging to the GH1 enzyme family, are reported. Both proteins have the (β/α)8 TIM-barrel fold, which is characteristic of this family and is also the most common protein structure. BglA is an octamer, whereas BglB is a monomer. Amyloid fibrillation using pH and temperature as perturbing agents was investigated using fluorescence spectroscopy as a preliminary approach and corroborated using wide-field optical microscopy, confocal microscopy, and field-emission scanning electron microscopy. These analyses showed that both enzymes fibrillate at a wide range of acidic and alkaline conditions and at several temperature conditions, particularly at acidic pH (3-4) and at temperatures between 45 and 65 °C. Circular dichroism spectroscopy corroborated the transition from an α-helix to a β-sheet secondary structure of both proteins in conditions where fibrillation was observed. Overall, our results suggest that fibrillation is a rather common phenomenon caused by protein misfolding, driven by a transition from an α-helix to a β-sheet secondary structure, that many proteins can undergo if subjected to conditions that disturb their native conformation.

TRPM3 in the eye and in the nervous system - from new findings to novel mechanisms

The calcium-permeable cation channel TRPM3 can be activated by heat and the endogenous steroid pregnenolone sulfate. TRPM3's best understood function is its role as a peripheral noxious heat sensor in mice. However, the channel is expressed in various tissues and cell types including neurons as well as glial and epithelial cells. TRPM3 expression patterns differ between species and change during development. Furthermore, a plethora of TRPM3 variants that result from alternative splicing have been identified and the majority of these isoforms are yet to be characterized. Moreover, the mechanisms underlying regulation of TRPM3 are largely unexplored. In addition, a micro-RNA gene (miR-204) is located within the TRPM3 gene. This complexity makes it difficult to obtain a clear picture of TRPM3 characteristics. However, a clear picture is needed to unravel TRPM3's full potential as experimental tool, diagnostic marker and therapeutic target. Therefore, the newest data related to TRPM3 have to be discussed and to be put in context as soon as possible to be up-to-date and to accelerate the translation from bench to bedside. The aim of this review is to highlight recent results and developments with particular focus on findings from studies involving ocular tissues and cells or peripheral neurons of rodents and humans.

The cryo-EM Structure of Thermotoga maritima β-Galactosidase: Quaternary Structure Guides Protein Engineering

Lactose intolerance is a common digestive disorder that affects a large proportion of the adult human population. The severity of the symptoms is highly variable, depending on the susceptibility to the sugar and the amount digested. For that reason, enzymes that can be used for the production of lactose-free milk and milk derivatives have acquired singular biotechnological importance. One such case is Thermotoga maritima β-galactosidase (TmLac). Here, we report the cryo-EM structure of TmLac at 2.0 Å resolution. The protein features a newly solved domain at its C-terminus, characteristic of the genus Thermotoga, which promotes a peculiar octameric arrangement. We have assessed the constraints imposed by the quaternary protein structure on the construction of hybrid versions of this GH2 enzyme. Carbohydrate binding modules (CBM) from the CBM2 and CBM9 families have been added at either the amino or carboxy terminus, and the structural and functional effects of such modifications have been analyzed. The results provide a basis for the rational design of hybrid enzymes that can be efficiently attached to different solid supports.

Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives

Congenital lactase deficiency (CLD) is a severe autosomal recessive genetic disorder that affects the functional capacity of the intestinal protein lactase-phlorizin hydrolase (LPH). This disorder is diagnosed already during the first few days of the newborn’s life due to the inability to digest lactose, the main carbohydrate in mammalian milk. The symptoms are similar to those in other carbohydrate malabsorption disorders, such as congenital sucrase-isomaltase deficiency, and include severe osmotic watery diarrhea. CLD is associated with mutations in the translated region of the LPH gene that elicit loss-of-function of LPH. The mutations occur in a homozygote or compound heterozygote pattern of inheritance and comprise missense mutations as well as mutations that lead to complete or partial truncations of crucial domains in LPH, such as those linked to the folding and transport-competence of LPH and to the catalytic domains. Nevertheless, the identification of the mutations in CLD is not paralleled by detailed genotype/protein phenotype analyses that would help unravel potential pathomechanisms underlying this severe disease. Here, we review the current knowledge of CLD mutations and discuss their potential impact on the structural and biosynthetic features of LPH. We also address the question of whether heterozygote carriers can be symptomatic for CLD and whether genetic testing is needed in view of the severity of the disease.

Structural determinants for transport of lactase phlorizin-hydrolase in the early secretory pathway as a multi-domain membrane glycoprotein

BACKGROUND

Lactase phlorizin-hydrolase (LPH) is a membrane anchored type I glycoprotein of the intestinal epithelium that is composed of four homologous structural domains. The role of each distinct domain in the intramolecular organization and function of LPH is not completely understood.

METHODS

Here, we analyzed the early events of LPH biosynthesis and trafficking by directed restructuring of the domain compositions.

RESULTS

Removal of domain I (LPH∆1) results in a malfolded ER-localized protein. By contrast, LPH without domain II (LPH∆2) is normally transported along the secretory pathway, but does not dimerize nor is enzymatically active. Interestingly a polypeptide stretch in domain II between L⁷³⁵-R⁸⁶⁸ exerts an intriguing role in modulating the trafficking behavior of LPH and its biological function. In fact, association of this stretch with transport-competent LPH chimeras results in their ER-arrest or aberrant trafficking. This stretch harbors a unique N-glycosylation site that is responsible for LPH retention in the ER via association with calnexin and facilitates proper folding of domains I and III before ER exit of LPH. Notably, a similar N-glycosylation site is also found in domain IV with comparable effects on the trafficking of LPH-derived molecules.

CONCLUSIONS

Our study provides novel insights into the intramolecular interactions and the sequence of events involved in the folding, dimerization and transport of LPH.

GENERAL SIGNIFICANCE

Elucidation of the structural-functional relevance of the domains in pro-LPH is crucial in unravelling and understanding the molecular basis of carbohydrate malabsorption disorders that are associated with lactase deficiency or lactase malfunction.

The Diverse Forms of Lactose Intolerance and the Putative Linkage to Several Cancers

Lactase-phlorizin hydrolase (LPH) is a membrane glycoprotein and the only β-galactosidase of the brush border membrane of the intestinal epithelium. Besides active transcription, expression of the active LPH requires different maturation steps of the polypeptide through the secretory pathway, including N- and O-glycosylation, dimerization and proteolytic cleavage steps. The inability to digest lactose due to insufficient lactase activity results in gastrointestinal symptoms known as lactose intolerance. In this review, we will concentrate on the structural and functional features of LPH protein and summarize the cellular and molecular mechanism required for its maturation and trafficking. Then, different types of lactose intolerance are discussed, and the molecular aspects of lactase persistence/non-persistence phenotypes are investigated. Finally, we will review the literature focusing on the lactase persistence/non-persistence populations as a comparative model in order to determine the protective or adverse effects of milk and dairy foods on the incidence of colorectal, ovarian and prostate cancers.

Immobilization of thermostable β-galactosidase on epoxy support and its use for lactose hydrolysis and galactooligosaccharides biosynthesis

Thermoresistant, recombinant β-galactosidase from Thermotoga maritima was purified and immobilized on the surface of epoxy-coated magnetic beads. The enzyme, which has hexameric quaternary structure as shown by gel filtration chromatography, attaches to the resin through multiple covalent linkages that involve different subunits. The bound enzyme shows higher stability than the free form. The immobilized enzyme showed to be efficient for the hydrolysis of lactose and the biosynthesis of galactooligosaccharides (GOS). The chemical structure of synthesized GOS has been determined by NMR revealing that the main product was β-3′-galactosyl lactose. Although β-galactosidases from different sources have been used for the same purposes, the distinct advantage of the methodology described in this communication is that the enzyme can be easily produced, purified and immobilized in large quantities.

Cloning and characterization of canine prostate-specific membrane antigen

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a promising biomarker in the diagnosis of prostate cancer and a potential target for antibody-based therapeutic strategies. We isolated the canine PSMA cDNA and investigated the cellular and biochemical characteristics of the recombinant protein as a potential target for animal preclinical studies of antibody based-therapies.

METHODS

Canine PSMA cDNA was isolated by PCR, cloned into expression vectors and transfected into COS-1 and MDCK cells. The biosynthesis and glycosylation of the recombinant protein were investigated in pulse-chase experiments, the cellular localization by confocal laser microscopy, the mode of association of PSMA with the membrane with solubilization in different detergents and its quaternary structure in sucrose-density gradients.

RESULTS

Canine PSMA shows 91% amino acid homology to human PSMA, whereby the major difference is a longer cytoplasmic tail of canine PSMA compared to its human counterpart. Canine PSMA is trafficked efficiently along the secretory pathway, undergoes homodimerization when it acquires complex glycosylated mature form. It associates with detergent-resistant membranes, which act as platforms along its intracellular trafficking. Confocal analysis revealed canine PSMA at the cell surface, Golgi, and the endoplasmic reticulum. A similar distribution is revealed for human PSMA, yet with reduced cell surface levels.

CONCLUSIONS

The cloning, expression, biosynthesis, processing and localization of canine PSMA in mammalian cells is described. We demonstrate that canine PSMA reveals similar characteristics to human PSMA rendering this protein useful as a translational model for investigations of prostate cancer as well as a suitable antigen for targeted therapy studies in dogs.

Impairment of protein trafficking by direct interaction of gliadin peptides with actin

Intestinal celiac disease (CD) is triggered by peptic-tryptic digest of gluten, known as Frazer's Fraction (FF), in genetically predisposed individuals. Here, we investigate the immediate effects of FF on the actin cytoskeleton and the subsequent trafficking of actin-dependent and actin-independent proteins in COS-1 cells. Morphological alterations in the actin filaments were revealed concomitant with a drastic reduction in immunoprecipitated actin from cells incubated with FF. These alterations elicit impaired protein trafficking of intestinal sucrase-isomaltase, a glycoprotein that follows an actin-dependent vesicular transport to the cell surface. However, the actin-independent transport of intestinal lactase phlorizin hydrolase remains unaffected. Moreover, the morphological alteration in actin is induced by direct interaction of this protein with gliadin peptides carrying the QQQPFP epitope revealed by co-immunoprecipitation utilizing a monoclonal anti-gliadin antibody. Finally, stimulation of cells with FF directly influences the binding of actin to Arp2. Altogether, our data demonstrate that FF directly interacts with actin and alters the integrity of the actin cytoskeleton thus leading to an impaired trafficking of intestinal proteins that depend on an intact actin network. This direct interaction could be related to the endocytic segregation of gliadin peptides as well as the delayed endocytic vesicle trafficking and maturation in gliadin-positive intestinal epithelial cells and opens new insights into the pathogenesis of CD.