Transglutaminase-2 interaction with heparin: identification of a heparin binding site that regulates cell adhesion to fibronectin-transglutaminase-2 matrix

Heparan sulphate proteoglycans (HSPG) are critical binding partners for extracellular tranglutaminase-2 (TG2), a multifunctional protein involved in tissue remodelling events related to organ fibrosis and cancer progression. We previously showed that TG2 has a strong affinity for heparan sulphate (HS)/heparin and reported that the HSPG syndecan-4 acts as a receptor for TG2 via its HS chains in two ways, by increasing TG2-cell surface trafficking/externalisation and by mediating RGD-independent cell adhesion to fibronectin-TG2 matrix during wound healing. Here we have investigated the molecular basis of this interaction. Site-directed mutagenesis revealed that either mutation of basic RRWK (262-265) or KQKRK (598-602) clusters, forming accessible heparin binding sequences on the TG2 three-dimensional structure, led to an almost complete reduction of heparin binding, indicating that both clusters contribute to form a single binding surface. Mutation of residues R(19) and R(28) also led to a significant reduction in heparin binding, suggesting their involvement. Our findings indicate that the heparin binding sites on TG2 mainly comprise two clusters of basic amino acids, which are distant in the linear sequence, but brought into spatial proximity in the folded ''closed'' protein, forming a high affinity heparin binding site. Molecular modelling showed that the identified site can make contact with a single heparin-derived pentasaccharide. The TG2-heparin binding mutants supported only weak RGD-independent cell adhesion compared to wild-type TG2 or mutants with retained heparin binding, and both heparin binding clusters were critical for TG2-mediated cell adhesion. These findings significantly advance our knowledge of how HS/heparin influences the adhesive function of TG2.