Structure of the C-Type Lectin Carbohydrate Recognition Domain of Human Tetranectin

Abstract

Tetranectin (TN) is a C-type lectin involved in fibrinolysis, being the only endogenous ligand known to bind specifically to the kringle 4 domain of plasminogen. TN was originally isolated from plasma, but shows a wide tissue distribution. Furthermore, TN has been found in the extracellular matrix of certain human carcinomas, whereas none or little is present in the corresponding normal tissue. The crystal structure of full-length trimeric TN (2.8 A resolution) has recently been published [Nielsen et al. (1997). FEBS Lett. 412, 388-396]. The crystal structure of the carbohydrate recognition domain (CRD) of human TN (TN3) has been determined separately at 2.0 A resolution in order to obtain detailed information on the two calcium binding sites. This information is essential for the elucidation of the specificity of TN towards oligosaccharides. TN3 crystallizes as a dimer, whereas it appears as a monomer in solution. The overall fold of TN3 is similar to other known CRDs. Each monomer is built of two distinct regions, one region consisting of six beta-strands and two alpha-helices, and the other region is composed of four loops harboring two calcium ions. The calcium ion at site 1 forms an eightfold coordinated complex and has Asp116, Glu120, Gly147, Glu150, Asn151, and one water molecule as ligands. The calcium ion at site 2, which is believed to be involved in recognition and binding of oligosaccharides, is sevenfold coordinated with ligands Gln143, Asp145, Glu150, Asp165, and two water molecules. One sulfate ion has been located at the surface of TN3, forming contacts to Glu120, Lys148, Asn106 of a symmetry-related molecule, and to an ethanol molecule.

Biopharmaceutical drug discovery using novel protein scaffolds

“…The CTLD structural units share a common structural core and serve as a scaffold to hold in place the more individual loop regions that line the ligand-binding site [12]. Diversity can be introduced by randomization of the sequences of individual loops, creating a much larger pool of potential binders than can be displayed on phage.…”

The heparin-binding site in tetranectin is located in the N-terminal region and binding does not involve the carbohydrate recognition domain

“…The Plg kringle-4-binding site is located within the domain encoded by exon 3 [13], which encodes a domain similar with the carbohydrate recognition domains (CRDs) of the C-type lectin superfamily [14]. TN exhibits pronounced structural similarity to both the neck region and the CRD of the collectins, and binds Ca# + in a similar way [15,16]. The binding to Plg kringle 4 has been shown to involve amino acid residues near and within the Ca# + -binding site 2 ; accordingly, Plg kringle 4 and Ca# + are competitive ligands for the CRD domain of TN [13].…”

Mass Spectrometric Characterisation of Post-Translational Modification and Genetic Variation in Human Tetranectin

“…Solution of the crystal structures of recombinant TN (Nielsen et al, 1997) and of the CRD domain of TN (Kastrup et al, 1998) showed that the structural element governing TN trimerisation is a short ␣-helical coiled coil and that the framework structure of TN CRD is almost super-imposable on framework structures of several other CRD domains, except in the loops that define the two calcium binding sites and the putative sugar binding site. Solution of the crystal structures of recombinant TN (Nielsen et al, 1997) and of the CRD domain of TN (Kastrup et al, 1998) showed that the structural element governing TN trimerisation is a short ␣-helical coiled coil and that the framework structure of TN CRD is almost super-imposable on framework structures of several other CRD domains, except in the loops that define the two calcium binding sites and the putative sugar binding site.…”