L-type lectins
L-type lectins |
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Domain organization and metal ion binding in
proteins containing L-type lectin-like domains
Structure of rat ERGIC-53 with bound Ca2+ ions
Beta-strands in one sheet are coloured dark blue and those in the opposing sheet are coloured yellow. The beta-hairpin structure is coloured blue and helices are coloured red. The Ca2+ ions are shown in pink. Protein Data Bank structure ID: 1R1Z. Structure of yeast Emp46p with bound K+ (2A6V)
Secondary structure elements are coloured as described above. The K+ ion is shown in pink. Protein Data Bank structure ID: 2A6V. |
Introduction The L-type CRD is named after the lectins found in abundance in the seeds of leguminous plants, such as concanavalin A from jack beans. The domain is present in plant, fungal, and animal proteins, but plant and animal L-type lectins have divergent sequences and different molecular properties: the plant lectins are secreted, soluble proteins and are found at high level in specialised tissues, while the animal L-type lectins are (often membrane-bound) luminal proteins and are found at low levels in many different cell types. These differences reflect the fact that plant and animal L-type lectins are likely to serve different functions. L-type lectins in animals L-type lectins in animal cells are involved in protein sorting in luminal compartments of animal cells. In humans and other mammals there are four L-type lectins: ERGIC-53, ERGL, VIP36, and VIPL. ERGL is found only in mammals and VIP36 is restricted to vertebrates, but ERGIC-53 and VIPL are also found in invertebrates. A protein similar to ERGIC-53 is present in the slime mold Dictyostelium dyscoideum, a very simple eukaryote. Proteins more distantly related to ERGIC-53 and VIP36 are present in yeast and other fungi and in protozoa. ERGIC-53 is a type I transmembrane protein with a single luminal L-type CRD and neck region containing a coiled-coil domain, named for its localization to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC), a stepping-stone in the trafficking of proteins from the ER to the Golgi. ERGIC-53 binds in a calcium-dependent manner to the high-mannose glycans borne by folded glycoproteins in the ER lumen which are ready to exit the ER. It transports such proteins to the ERGIC, where the slightly lower pH triggers dissociation of receptor and cargo, allowing the receptor to recycle back to the ER. ERGIC-53 is only essential for the trafficking of a subset of glycoproteins. It is known to assist the ER exit of cathepsins C and Z and blood coagulation factors V and VIII - mutations in the human ERGIC-53 gene cause a clotting deficiency as a result of the reduced secretion of these clotting factors into serum. ERGL (ERGIC-53-like) is a mammalian protein related to ERGIC-53, which interestingly lacks a number of key residues identified in other animal L-type lectins as being critical for calcium- or sugar-binding activity. Expression of both human and rat ERGL (termed SLAMP) is restricted to a small number of specific tissues and cell types, where the protein is localized to the ERGIC, suggesting a specialized role in assisting the secretion of specific glycoproteins. VIP36 (vesicular integral membrane protein of 36 kDa) is a type I transmembrane protein with a single luminal L-type CRD, which cycles between the ER and the Golgi. VIP36 exhibits shared sugar-binding specificity with ERGIC-53: it binds with highest affinity to high mannose N-linked glycans of the type found on glycoproteins which are correctly folded and have been processed by the glycosidases of the ER but not those of the Golgi. Glycan binding is also calcium-dependent; VIP36 possesses two Ca2+ binding sites, and exhibits conservation of the site 2 calcium coordinating residue with ERGIC-53. VIP36 shows a different pH-dependence of ligand binding to ERGIC-53, exhibiting optimum ligand binding at pH 6.5, at which pH level ERGIC-53 releases ligands, with reduced binding at more or less acidic conditions. VIP36 is therefore likely to pick up ligand at a later point in the secretory pathway (which becomes increasingly acidic due to inward pumping of H+) than ERGIC-53. VIP36 may function to traffic proteins from the ERGIC to the cis-Golgi, or may mediate retrograde transport of glycoproteins which have escaped correct glycan trimming and modification. VIPL (VIP36-like) is very similar to VIP36 and the two proteins probably arose by gene duplication in an early vertebrate ancestor. VIPL is suggested to be the orthologue of the ancestral VIP36/VIPL protein, but was discovered after VIP36; previously identified VIP36 homologues in non-vertebrates have therefore here been reclassified as VIPL proteins. A further gene duplication has resulted in fish possessing two VIPL proteins plus one VIP36. It is not clear whether VIPL cycles between the ER and the ERGIC/cis-Golgi, or whether it is confined to the ER. VIPL is reported to enhance secretion of a subset of glycoproteins and may function as a regulator of ERGIC-53. L-type lectins in simpler eukaryotes Emp46p and Emp47p are L-type lectins from Saccharomyces cerevisiae which cycle between the ER and the Golgi to facilitate the exit of N-linked glycoproteins from the ER. Unlike ERGIC-53, binding of high mannose glycans does not require a Ca2+ ion. Emp46p binds a K+ ion, which is essential for glycoprotein transport, at a different location to that of the Ca2+ ion in ERGIC-53, and Emp47p does not bind any metal ions. The differences in metal binding are evident in the primary structure of the proteins. Structure and sugar binding The L-type CRD is a beta-sandwich structure with a concave sheet of seven beta-strands and a convex sheet of five. The ligand binding site is in a negatively-charged cleft of conserved residues. Most L-type CRDs require metal ions for ligand binding: in concanavalin A, a transition metal is bound at one site and Ca2+ at the second site. Despite divergent sequences and function, it seems likely that the L-type CRDs have retained similar mechanisms of sugar binding. Certain key residues in four loop regions that contribute to the binding sites in the plant proteins are conserved in all of the animal and plant L-type CRDs. |
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