title: 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.
abstract: BACKGROUND
  Holo-(acyl carrier protein) synthase (AcpS), a member of the phosphopantetheinyl transferase superfamily, plays a crucial role in the functional activation of acyl carrier protein (ACP) in the fatty acid biosynthesis pathway. AcpS catalyzes the attachment of the 4'-phosphopantetheinyl moiety 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 A. We also have determined the structures of AcpS alone (at 1.8 A) and AcpS in complex with CoA (at 1.5 A). These structures reveal that AcpS exists as a trimer. A catalytic center is located at each of the solvent-exposed interfaces between AcpS molecules. Site-directed mutagenesis studies confirm 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 form 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 transferases.
authors.affiliation: Biological Chemistry, Wyeth-Ayerst Research, Cambridge, MA 02140, USA. parrisk@war.wyeth.com
authors.forename: K D
authors.initials: KD
authors.lastname: Parris
authors.forename: L
authors.initials: L
authors.lastname: Lin
authors.forename: A
authors.initials: A
authors.lastname: Tam
authors.forename: R
authors.initials: R
authors.lastname: Mathew
authors.forename: J
authors.initials: J
authors.lastname: Hixon
authors.forename: M
authors.initials: M
authors.lastname: Stahl
authors.forename: C C
authors.initials: CC
authors.lastname: Fritz
authors.forename: J
authors.initials: J
authors.lastname: Seehra
authors.forename: W S
authors.initials: WS
authors.lastname: Somers
mesh_terms: D000213:Acyl Carrier Protein / Q000737:chemistry / Q000378:metabolism; D000595:Amino Acid Sequence; D001412:Bacillus subtilis / Q000737:chemistry / Q000201:enzymology*; D001665:Binding Sites; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016415:Sequence Alignment; D013379:Substrate Specificity; D017855:Transferases (Other Substituted Phosphate Groups) / Q000737:chemistry* / Q000378:metabolism
chemical_list: D000213:Acyl Carrier Protein; D017855:Transferases (Other Substituted Phosphate Groups); C021364:holo-(acyl-carrier-protein) synthase