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

SlpA (SlyD-like protein A) comprises two domains, a FK506 binding domain (FKBP fold) of moderate prolyl <i>cis</i>/<i>trans</i>-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 <i>Escherichia coli</i> 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