Folding and stability of the active N-terminal domain of tissue inhibitor of metalloproteinases-1 and -2.

The truncated forms of tissue inhibitor of metalloproteinase-1 and -2 (delta TIMP-1 and -2), comprising the N-terminal active domain, are ideal molecules for structural analysis by intrinsic fluorescence as each contains a single conserved tryptophan residue. In this paper we describe studies on their conformational stability, unfolding/refolding kinetics and the environment of the unique tryptophan as judged by its fluorescence properties in the native state and exposure to an external quencher, acrylamide. Two forms of delta TIMP-2 were studied: delta TIMP-2 T21 derived from the full-length cDNA clone isolated from a mixed-tumour library, and delta TIMP-2 A21 containing the highly conserved V18IRAK22 sequence. In all three delta TIMP proteins the tryptophan environments in the native state appeared to be similar, but substantial differences were seen in their conformational stabilities and refolding kinetics. delta TIMP-1 was approximately twice as stable as delta TIMP-2 T21 and 1.4-fold more stable than delta TIMP-2 A21. This stability difference between delta TIMP-1 and delta TIMP-2 was shown to be independent of N-linked glycosylation. delta TIMP-1 and delta TIMP-2 A21 both showed simple two-state refolding kinetics, whereas delta TIMP-2 T21 refolding was more complex and biphasic in character. These differences between delta TIMP-2 T21 and A21 suggest that residue 21 is a structurally important site in the TIMP protein. All three truncated molecules can be considered as stable independent folding domains ideally suited for further structural analysis.