Mannose-6-phosphate receptors (MPR 300 and 46) from the highly evolved invertebrate Asterias rubens (Echinodermate): biochemical and functional characterization of MPR 46 protein

Biochemical Characterization of a Lysosomal α-Mannosidase from the Starfish Asterias rubens

Acidic α-mannosidase is an important enzyme and is reported from many different plants and animals. Lysosomal α-mannosidase helps in the catabolism of glycoproteins in the lysosomes thereby playing a major role in cellular homeostasis. In the present study lysosomal α-mannosidase from the gonads of echinoderm Asterias rubens was isolated and purified. The crude protein sample from ammonium sulfate precipitate contained two isoforms of mannosidase as tested by the MAN2B1 antibody, which were separated by anion exchange chromatography. Enzyme with 75 kDa molecular weight was purified and biochemically characterized. Optimum pH of the enzyme was found to be in the range of 4.5-5 and optimum temperature was 37 °C. The activity of the enzyme was inhibited completely by swainsonine but not by 1-deoxymannojirimycin. Ligand blot assays showed that the enzyme can interact with both the lysosomal enzyme sorting receptors indicating the presence of mannose 6-phosphate in the glycan surface of the enzyme. This is the first report of lysosomal α-mannosidase in an active monomeric form. Its interaction with the receptors suggest that the lysosomal enzyme targeting in echinoderms might follow a mannose 6-phosphate mediated pathway similar to that in the vertebrates.

Mannose 6-phosphate-dependent lysosomal enzyme targeting in hydra: a biochemical, immunological and structural elucidation

Mannose 6-phosphate (M6P)-dependent lysosomal enzyme targeting to endosome/lysosome complex is poorly understood among lower invertebrates. So far, only a M6P-independent lysosomal enzyme sorting protein, named LERP, has been described in Drosophila. Here, we have identified mannose 6-phosphate receptor (MPR) homologues in Hydra vulgaris, a basal Cnidarian, at genome level and further purified a cation-dependent MPR-like protein from hydra using affinity chromatography. Structural comparisons of hydra MPRs with mammalian MPRs confirm that the residues important for interacting with the M6P ligand are conserved. Based on our results, we report for the first time the occurrence of MPR-related proteins and M6P-dependent lysosomal enzyme targeting in H. vulgaris.

Unravelling novel functions of the endosomal transporter mannose 6-phosphate/insulin-like growth factor receptor (CD222) in health and disease: An emerging regulator of the immune system

Properly balanced cellular responses require both the mutual interactions of soluble factors with cell surface receptors and the crosstalk of intracellular molecules. In particular, immune cells exposed unceasingly to an array of positive and negative stimuli must distinguish between what has to be tolerated and attacked. Protein trafficking is one of crucial pathways involved in this labour. The approximately >270-kDa protein transporter called mannose 6- phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) is a type I transmembrane glycoprotein present largely intracellularly in the Golgi apparatus and endosomal compartments, but also at the cell surface. It is expressed ubiquitously in a vast majority of higher eukaryotic cell types. Through binding and trafficking multiple unrelated extracellular and intracellular ligands, CD222 is involved in the regulation of a plethora of functions, and thus implicated in many physiological but also pathophysiological conditions. This review describes, first, general features of CD222, such as its evolution, genomic structure and regulation, protein structure and ligands; and second, its specific functions with a special focus on the immune system.

P-Type Lectins: Cation-Dependent Mannose-6-Phosphate Receptor

In eukaryotic cells, post-translational modification of secreted proteins and intracellular protein transport between organelles are ubiquitous features. One of the most studied systems is the N-linked glycosylation pathway in the synthesis of secreted glycoproteins (Schrag et al. 2003). The N-linked glycoproteins are subjected to diverse modifications and are transported through ER and Golgi apparatus to their final destinations in- and outside the cell. Incorporation of cargo glycoproteins into transport vesicles is mediated by transmembrane cargo receptors, which have been identified as intracellular lectins. For example, mannose 6-phosphate receptors (Ghosh et al. 2003) function as a cargo receptor for lysosomal proteins in the trans-Golgi network, whereas ERGIC-53 (Zhang et al. 2003) and its yeast orthologs Emp46/47p (Sato and Nakano 2002) are transport lectins for glycoproteins that are transported out of ER.

Role of cation independent mannose 6-phosphate receptor protein in sorting and intracellular trafficking of lysosomal enzymes in chicken embryonic fibroblast (CEF) cells

Delivery of soluble lysosomal proteins to the lysosomes is dependent primarily on the mannose 6-phosphate receptors (MPRs) in mammals. However, in non-mammalian cells the role of MPR300 in sorting and trafficking of acid hydrolases to lysosomes is not fully understood till now. In this paper, we tested the role of MPR300 in sorting and trafficking of lysosomal enzymes in CEF cells using a small interfering RNA (siRNA) technology. Inactivation of MPR300 resulted in the secretion of large amounts of newly synthesized hydrolases into the medium and also inhibited the endocytosis of mannose 6-phospharylated ligands. Knockdown of MPR300 in CEF cells results in missorting of fucosidase and arylsulfatse A enzymes into the medium. The results indicated that the MPR300 in CEF cells plays a key role in sorting and trafficking of these soluble hydrolases.