Identification of the first archaeal oligopeptide-binding protein from the hyperthermophile Aeropyrum pernix

The archaeon Aeropyrum pernix grows optimally at 90_C and derives energy primarily from aerobic degradation of complex proteinaceous substrates. The ability of these nutrients to sustain growth is generally associated with the presence of oligopeptide transport systems, such as the well-known protein-dependent ATP-binding cassette (ABC) transporters. This study is concerned with the isolation and characterisation of the first archaeal oligopeptide-binding protein (OppAAp) from the extracellular medium of A. pernix. The protein shows a pI of 3.9 and a molecular mass of about 90 kDa under native conditions. By using a proteomic approach, the OppAAp-encoding gene was identified (APE1583) and about 55% of the protein amino-acid sequence was validated. The extracellular purified protein was able to efficiently bind oligopeptide substrates such as Xenopsin. The amount of a liganded peptide to OppAAp was about 70% at 90_C using a 1/100 (w/w) OppAAp/substrate ratio. Sequence comparisons showed a weak but significant similarity of OppAAp with bacterial oligopeptide binding proteins. Furthermore, APE1583 neighbouring genes encode for the cognate components of an ABC transport system, suggesting that these ORFs are organised in an operon-like structure, with OppAAp as the extracellular component for the uptake of oligopeptides.