Resonance assignment of the outer membrane protein AlkL in lipid bilayers by proton-detected solid-state NMR

Most commonly small outer membrane proteins, possessing between 8 and 12 β-strands, are not involved in transport but fulfill diverse functions such as cell adhesion or binding of ligands. An intriguing exception are the 8-stranded β-barrel proteins of the OmpW family, which are implicated in the transport of small molecules. A representative example is AlkL from Pseudomonas putida GPoI, which functions as a passive importer of hydrophobic molecules. This role is of high interest with respect to both fundamental biological understanding and industrial applications in biocatalysis, since this protein is frequently utilized in biotransformation of alkanes. While the transport function of AlkL is generally accepted, a controversy in the transport mechanism still exists. In order to address this, we are pursuing a structural study of recombinantly produced AlkL reconstituted in lipid bilayers using solid-state NMR spectroscopy. In this manuscript we present ¹H, ¹³C and ¹⁵N chemical shift assignments obtained via a suite of 3D experiments employing high magnetic fields (1 GHz and 800 MHz) and the latest magic-angle spinning (MAS) approaches at fast (60-111) kHz rates. We additionally analyze the secondary structure prediction in comparison with those of published structures of homologous proteins.