Novel mutant human fibronectin FIII9-10 domain pair with increased conformational stability and biological activity

The ninth and tenth type III domains (FIII9-10) in the central cell binding domain of human fibronectin contain integrin receptor binding sites, including RGD in FIII10 and a synergy site, PHSRN, in FIII9. The specific amino acids that contribute to cell binding have been identified by the use of wild-type and mutant fragments of human fibronectin containing the FIII9-10 domain pair. At high concentrations FIII9-10 mimics, to a large extent, the biological activity of the full-length fibronectin molecule. However, FIII9 is conformationally unstable, even in the context of the FIII9-10 pair. Here we report the construction of a series of hybrid mouse-human FIII9-10 pairs that confer varying degrees of conformational stability to FIII9. The conformational stability of the human FIII9 module was increased 2-3-fold by substitution of Leu1408 with Pro. We demonstrate that the biological activity of this mutant is enhanced. The resulting FIII9-10 mutant has good solution properties and will provide a template into which further mutations can be incorporated in order to probe the structure-function relationship of the cell binding module of fibronectin.