Crystal structures of substrate binding to Bacillus subtilis holo-(acyl carrier protein) synthase reveal a novel trimeric arrangement of molecules resulting in three active sites

Background: Holo-(acyl carrier protein) synthase (AcpS), a member of the ph osphopantetheinyl transferase superfamily, plays a crucial role in the func tional activation of acyl carrier protein (ACP) in the fatty acid biosynthe sis pathway. AcpS catalyzes the attachment of the 4'-phosphopantetheinyl mo iety of coenzyme A (CoA) to the sidechain of a conserved serine residue on apo-ACP. Results: We describe here the first crystal structure of a type II ACP from Bacillus subtilis in complex with its activator AcpS at 2.3 Angstrom. We a lso have determined the structures of AcpS alone (at 1.8 Angstrom) and AcpS in complex with CoA (at 1.5 Angstrom). These structures reveal that AcpS e xists as a trimer, A catalytic center is located at each of the solvent-exp osed interlaces between AcpS molecules. Site-directed mutagenesis studies c onfirm the importance of trimer formation in AcpS activity. Conclusions: The active site in AcpS is only formed when two AcpS molecules dimerize, The addition of a third molecule allows for the formation of two additional active sites and also permits a large hydrophobic surface from each molecule of AcpS to be buried in the trimer. The mutations Ile5-->Arg, Gln113-->Glu and Gln113-->Arg show that AcpS is inactive when unable to fo rm a trimer, The co-crystal structures of AcpS-CoA and AcpS-ACP allow us to propose a catalytic mechanism for this class of 4'-phosphopantetheinyl tra nsferases.