Structure-Based Insights into the Dynamics and Function of Two-Domain SlpA from Escherichia coli

SlpA (SlyD-like protein A) comprises two domains, a FK506 binding domain (FKBP fold) of moderate prolyl cis/trans-isomerase activity and an inserted in flap (IF) domain that hosts its chaperone activity. Here we present the nuclear magnetic resonance (NMR) solution structure of apo Escherichia coli SlpA determined by NMR that mirrors the structural properties seen for various SlyD homologues. Crucial structural differences in side-chain orientation arise for F37, which points directly into the hydrophobic core of the active site. It forms a prominent aromatic stacking with F15, one of the key residues for PPIase activity, thus giving a possible explanation for the inherently low PPIase activity of SlpA. The IF domain reveals the highest stability within the FKBP-IF protein family, most likely arising from an aromatic cluster formed by four phenylalanine residues. Both the thermodynamic stability and the PPIase and chaperone activity let us speculate that SlpA is a backup system for homologous bacterial systems under unfavorable conditions.