Alanine scanning mutagenesis and functional analysis of the fibronectin-like collagen-binding domain from human 92-kDa type IV collagenase

The human 72-kDa (CLG4A) and 92-kDa (CLG4B) type IV collagenases contain a domain consisting of three contiguous copies of the fibronectin (FN)-derived type II homology unit (T2HU), T2HU-1, T2HU-2, and T2HU-3. To investigate the functional role of this domain, we have constructed plasmids expressing β-galactosidase fusion proteins with one or more of the CLG4B-derived T2HU. The gelatin binding assays demonstrate that a single copy of T2HU-2 renders β-galactosidase capable of binding gelatin. The three repeats, however, differ dramatically in their capacity to bind gelatin, with T2HU-1 and T2HU-3 having significantly less binding activity than T2HU-2. Using alanine scanning mutagenesis we have defined the amino acid residues (Arg³⁰⁷, Asp³⁰⁹, Asn³¹⁹, Tyr³²⁰, Asp³²³) that are critical for gelatin binding of T2HU-2. The low gelatin binding of T2HU-1 compared to T2HU-2 was traced to the non-conserved residues Ala²²⁸-Ala and Leu²⁵³-Pro. The results suggest that the gelatin binding of the type IV collagenase proenzyme is mediated by the FN-like domain, although the presence of another gelatin-binding site cannot be excluded. The FN domain-mediated binding, however, is not a rate-limiting step in the hydrolysis of gelatin by the enzyme.