Folding of a synthetic parallel β‐roll protein

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Abstract
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.