Recently, the design of β-sheet proteins and concomitant folding studies have attracted increasing attention. A unique natural all-β domain occurs in a family of cytolytic bacterial toxins, the so-called RTX toxins. This domain consists of a variable number (about 6–45) of tandem repeats of a glycine-rich nine-residue motif with the consensus sequence GGXGXDX(L/I/F)X. The analysis of the three-dimensional structure of alkaline protease from Pseudomonas aeruginosa which possesses six of these repeats revealed that they fold into a novel 'parallel β-roll’ where calcium is bound within the turns connecting the β-strands. A 75-mer peptide of the sequence NH2-WLS-[GGSGNDNLS]8-COOH was chemically synthesised. Circular dichroism spectroscopy showed that this polypeptide folds in the presence of Ca2+ and polyethylene glycol into a β-structure which is presumably identical with the parallel β-roll. This synthetic β-roll behaves similarly to the isolated β-roll domains from Escherichia coli haemolysin or Bordetella pertussis cyclolysin in terms of calcium binding and polymerisation behaviour.
