Laminin G1 residues of protein S mediate its TFPI cofactor function and are competitively regulated by C4BP.

Protein S is a cofactor in the tissue factor pathway inhibitor (TFPI) anticoagulant pathway. It enhances TFPIα-mediated inhibition of factor (F)Xa activity and generation. The enhancement is dependent on a TFPIα-protein S interaction, involving TFPIα Kunitz 3 and protein S laminin G-type (LG)-1. C4b binding protein (C4BP), which binds to protein S LG1, almost completely abolishes its TFPI cofactor function. However, neither the amino acids involved in TFPIα enhancement, nor the mechanisms underlying the reduced TFPI cofactor function of C4BP-bound protein S, are known. To screen for functionally important regions within protein S LG1 we generated seven variants with inserted N-linked glycosylation attachment sites. Protein S D253T and Q427N/K429T, displayed severely reduced TFPI cofactor function while showing normal activated protein C (APC) cofactor function and C4BP binding. Based on these results, we designed four protein S variants in which 4-6 surface exposed charged residues were substituted for alanine. One variant, protein S K255A/E257A/D287A/R410A/K423A/E424A, exhibited either abolished or severely reduced TFPI cofactor function in plasma and FXa inhibition assays, both in the presence or absence of FV-short, but retained normal APC cofactor function and high affinity C4BP-binding. The C4BP β-chain was expressed to determine the mechanisms behind the reduced TFPI cofactor function of C4BP-bound protein S. Like C4BP-bound protein S, C4BP β-chain-bound protein S had severely reduced TFPI cofactor function. These results show that protein S Lys255, Glu257, Asp287, Arg410, Lys423 and Glu424 are critical for protein S-mediated enhancement of TFPIα and that binding of the C4BP β-chain blocks this function

The first laminin G-like domain of protein S is essential for binding and activation of Tyro3 receptor and intracellular signalling

The homologous proteins Gas6 and protein S (ProS1) are both natural ligands for the TAM (Tyro3, Axl, MerTK) receptor tyrosine kinases. ProS1 selectively activates Tyro3; however, the precise molecular interface of the ProS1-Tyro3 contact has not been characterised. We used a set of chimeric proteins in which each of the C-terminal laminin G-like (LG) domains of ProS1 were swapped with those of Gas6, as well as a set of ProS1 mutants with novel added glycosylations within LG1. Alongside wildtype ProS1, only the chimera containing ProS1 LG1 domain stimulated Tyro3 and Erk phosphorylation in human cancer cells, as determined by Western blot. In contrast, Gas6 and chimeras containing minimally the Gas6 LG1 domain stimulated Axl and Akt phosphorylation. We performed in silico homology modelling and molecular docking analysis to construct and evaluate structural models of both ProS1-Tyro3 and Gas6-Axl ligand-receptor interactions. These analyses revealed a contact between the ProS1 LG1 domain and the first immunoglobulin domain of Tyro3, which was similar to the Gas6-Axl interaction, and involved long-range electrostatic interactions that were further stabilised by hydrophobic and polar contacts. The mutant ProS1 proteins, which had added glycosylations within LG1 but which were all outside of the modelled contact region, all activated Tyro3 in cells with no hindrance. In conclusion, we show that the LG1 domain of ProS1 is necessary for activation of the Tyro3 receptor, involving protein-protein interaction interfaces that are homologous to those of the Gas6-Axl interaction

Exosites in hypervariable loops of ADAMTS dpacer domains control substrate recognition and proteolysis

ADAMTS (A Disintegrin-like and Metalloproteinase domain with Thrombospondin type 1 Motif)-1, -4 and -5 share the abilities to cleave large aggregating proteoglycans including versican and aggrecan. These activities are highly relevant to cardiovascular disease and osteoarthritis and during development. Here, using purified recombinant ADAMTS-1, -4 and -5, we quantify, compare, and define the molecular basis of their versicanase activity. A novel sandwich-ELISA detecting the major versican cleavage fragment was used to determine, for the first time, kinetic constants for versican proteolysis. ADAMTS-5 (kcat/Km 35 × 105 M−1 s−1) is a more potent (~18-fold) versicanase than ADAMTS-4 (kcat/Km 1.86 × 105 M−1 sec−1), whereas ADAMTS-1 versicanase activity is comparatively low. Deletion of the spacer domain reduced versicanase activity of ADAMTS-5 19-fold and that of ADAMTS-4 167-fold. Co-deletion of the ADAMTS-5 cysteine-rich domain further reduced versicanase activity to a total 153-fold reduction. Substitution of two hypervariable loops in the spacer domain of ADAMTS-5 (residues 739–744 and 837–844) and ADAMTS-4 (residues 717–724 and 788–795) with those of ADAMTS-13, which does not cleave proteoglycans, caused spacer-dependent reductions in versicanase activities. Our results demonstrate that these loops contain exosites critical for interaction with and processing of versican. The hypervariable loops of ADAMTS-5 are shown to be important also for its aggrecanase activity. Together with previous work on ADAMTS-13 our results suggest that the spacer domain hypervariable loops may exercise significant control of ADAMTS proteolytic activity as a general principle. Identification of specific exosites also provides targets for selective inhibitors