A circular loop of the 16-residue repeating unit in ice nucleation protein

Ice nucleation protein (INP) from Gram-negative bacteria promotes the freezing of supercooled water. The central domain of INPs with 1034–1567 residues consists of 58–81 tandem repeats with the 16-residue consensus sequence of AxxxSxLTAGYGSTxT. This highly repetitive domain can also be represented by tandem repeats of 8-residues or 48-residues. In order to elucidate the structure of the tandem repeats, NMR measurements were made for three synthetic peptides including QTARKGSDLTTGYGSTS corresponding to a section of the repetitive domains in Xanthomonas campestris INP. One remarkable observation is a long-range NOE between the side chains of Tyr(i) and Ala(i-10) in the 17-residue peptide. Medium-range NOEs between the side chains of Tyr(i) and Leu(i-4), Thr(i-3) or Thr(i-2) were also observed. These side chain-side chain interactions can be ascribed to CH/π interaction. Structure calculation reveals that the 17-residue peptide forms a circular loop incorporating the 11-residue segment ARKGSDLTTGY.