EFFECTS OF PROLINE CIS-TRANS ISOMERIZATION ON TB DOMAIN SECONDARY STRUCTURE

The transforming growth factor beta (TGF-beta) binding protein-like (T B) domain is found principally in proteins localized to extracellular matrix fibrils, including human fibrillin-1, the defective protein in the Marfan syndrome. Analysis of the nuclear magnetic resonance (NMR) data for the sixth TB module from human fibrillin-1 has revealed the e xistence of two stable conformers that differ in the isomerization sta tes of two proline residues. Unusually, the two isoforms do not readil y interconvert and are stable on the time scale of milliseconds. We ha ve computed independent structures of the major and miner conformers o f TB6 to assess how the domain fold adjusts to incorporate alternative ly cis- or trans-prolines. Based on previous observations, it has been suggested that multiple conformers can only be accommodated in flexib le regions of protein structure. Tn contrast, P22, which exists in tra ns in the major form and cis in the minor form of TB6, is in a rigid r egion of the domain, which is confirmed by backbone dynamics measureme nts. Overall, the structures of the major and minor conformers are sim ilar. However, the secondary structure topologies of the two forms dif fer as a direct consequence of the changes in proline conformation.