Interaction of Fibronectin with Tenascin

Tenascin-C (Tn) is the best characterized member of the tenascin family of cell-adhesion proteins [ref]. It interacts with integrins, collagens, proteoglycans and Fn [ref1,ref2]. Depending on the cellular context, it can function as an adhesive or antiadhesive protein and in soluble form can interfere with adhesion and spreading of fibroblasts on Fn-coated surfaces [ref]. Tn has a modular composition that includes a string of 14 EGF-like modules followed by 8 or more FnIII modules depending on species and splice variations. The 3rd FnIII module contains an RGD sequence that is recognized by some integrins [ref]. The alternately spliced region contains a site that binds with a Kd of ~ 2nM to annexin II on the surface of endothelial cells [ref]. The carboxy terminus contains a single module that is homologous to those found in the D region of fibrinogen. The amino-terminus contains cysteine residues that form interchain disulfide bonds resulting in a six-chain spider-like structure termed "hexabrachion" . Thus all variants of Tn are potentially hexavalent with respect to their interaction with ligands. The shorter forms bind much more effectively to Fn and preferentially co-distribute with Fn fibrils in the extracellular matrix suggesting that the Fn binding site is masked in the longer forms [ref]. The binding site for Fn was reported to be in the 3rd FnIII module of Tn since recombinant fragments containing this module are able to bind much better than those lacking it. However, the mM concentrations of these fragments required to compete for binding of Tn to solid phase Fn were much higher than the nM levels required for direct binding of these same fragments to solid phase Fn.

The location of the Tn binding site on Fn is controversial. Tn-C was reported to bind to module III-13 in the Hep-2 region of Fn, thereby competing with syndecan-4 and blocking signals for cell adhesion [ref]. This is in conflict with a more recent study that failed to detect an interaction with the Hep-2 fragment but instead produced evidence for a cryptic binding site in the N-terminal fib-1/hep-1 domain with a Kd ~ 1 mM [ref]. What is most clear is that additional studies are required. Measurements of interactions between purified fragments may not reveal all of what happens in vivo where these two proteins are multivalent and exist primarily in the solid phase.

Heparin binds to two regions of Tn, one within FnIII modules 3-5 and a second stronger one in the C-terminal fibrinogen-like module [ref]. Soluble glycosaminoglycans bind Tn and inhibit its binding to purified Fn on plastic [ref]. Perlecan, a large heparan sulfate proteoglycan, binds via its core protein to Fn [ref], co-localizes with Fn fibrils in the matrix and seems to enhance the binding of Tn to those fibrils, presumably via its GAG moiety serving as an additional bridge between Fn and Tn [ref].