COMMON METAL-ION COORDINATION IN LIM DOMAIN PROTEINS

The LIM motif is a cysteine- and histidine-rich sequence that was first identified in proteins involved in control of gene expression and cell differentiation. In order to characterize structural features of the LIM domain, we have carried out biophysical studies on two polypeptides that display LIM domains: the cysteine-rich intestinal protein (CRIP) and a fragment of the cysteine-rich protein (CRP). Bacterial expression vectors were constructed for the intact CRIP molecule and the C-terminal half of CRP, designated LIM2, such that each expressed protein contained a single LIM domain. Both proteins were recovered as soluble, Zn(Il)-containing proteins. The metal coordination properties of these two distinct LIM domain proteins were highly similar, suggesting that a common structural architecture may exist in LIM domain proteins. Both proteins exhibit a maximum of two tetrahedrally bound Zn(II) ions per molecule. Electronic spectroscopy of Co(II) complexes and Cd-113 NMR of Cd(II) complexes of CRIP and LIM2 revealed a similar ligand field pattern with one tetrathiolate (S4) site and one S3N1 site for divalent metal ions. The nitrogen ligand was shown to arise from a histidyl imidazole by heteronuclear multiple quantum coherence NMR. The eight conserved residues within the LIM domains of CRIP and LIM2 include seven cysteines and one histidine. It is likely that these conserved residues generate the S4 and S3N1 Zn(Il)-binding sites. Metal binding to the two sites within a single LIM domain is sequential, with preferential occupancy of the S4 site. Slow metal ion exchange occurs between sites within an LIM domain, and metal exchange with exogenous metal ions is observed, with exchange at the S3N1 site being kinetically more facile. In the absence of metal binding both proteins appear to be substantially unfolded. Metal binding stabilizes a tertiary fold containing appreciable secondary structural elements. The common metal ion coordination in CRIP and LIM2 suggests that the LIM motif may constitute a structural module with conserved features.