Structural Characterization of an ACP from : Insights into Hyperthermal Adaptation.

, a deep-branching hyperthermophilic bacterium, expresses an extraordinarily stable acyl carrier protein (-ACP) that functions as a carrier in the fatty acid synthesis system at near-boiling aqueous environments. Here, to understand the hyperthermal adaptation of -ACP, we investigated the structure and dynamics of -ACP by nuclear magnetic resonance (NMR) spectroscopy. The melting temperature of -ACP (101.4 °C) far exceeds that of other ACPs, owing to extensive ionic interactions and tight hydrophobic packing. The D59 residue, which replaces Pro/Ser of other ACPs, mediates ionic clustering between helices III and IV. This creates a wide pocket entrance to facilitate the accommodation of long acyl chains required for hyperthermal adaptation of the cell membrane. -ACP is revealed to be the first ACP that harbor an amide proton hyperprotected against hydrogen/deuterium exchange for I15. The hydrophobic interactions mediated by I15 appear to be the key driving forces of the global folding process of -ACP. Our findings provide insights into the structural basis of the hyperthermal adaptation of ACP, which might have allowed to survive in hot ancient oceans.