Structural and functional insights into Aeropyrum pernix OppA, a member of a novel archaeal OppA subfamily.

In this study we gain insight into the structural and functional characterization of the oligopeptide-binding protein (OppAAp) previously identified from the extracellular media of Aeropyrum pernix cell culture at late stationary phase. OppAAp showed the N-terminus Q32 in a pyroglutamate form and a C-terminal processing at the level of a threonine rich region, probably involved in protein membrane anchoring. Moreover, the medium - released OppAAp was identified as a 'nicked' form composed of two fragments tightly associated, detachable only under strong denaturing conditions. The cleavage site E569-G570, seems be located on an exposed surface-loop, highly conserved in several 3D structures of di/oligopeptide binding proteins from different sources. Structural and biochemical properties of the 'nicked' protein were virtually indistinguishable from those of the intact form. Indeed studies on the entire bacterial-expressed OppAAp owning the same N- and C-terminus of the 'nicked' form, supported these findings. Moreover, in the middle exponential growth phase, OppAAp was found as intact cell membrane-associated protein. Interestingly, the native exoprotein OppAAp was co-purified with an hexapeptide (EKFKIV) showing both lysines methylated and possibly originated from an A. pernix endogenous stress-induced lipoprotein. Therefore, the involvement of OppAAp in recycling endogenous proteins, was suggested as potential physiological function. Finally, a new OppA from Sulfolobus solfataricus, SSO1288, was purified and preliminary characterized, allowing to identify a common structural/genetic organization shared by all the "true" archaeal OppAs of the di/oligopeptide class.