Biotinylation in the hyperthermophile Aquifex aeolicus -: Isolation of a cross-linked BPL:BCCP complex

Biotin protein ligase (BPL) catalyses the biotinylation of the biotin carboxyl carrier protein (BCCP) subunit of acetyl CoA carboxylase and this post-translational modification of a single lysine residue is exceptionally specific. The exact details of the protein-protein interactions involved are unclear as a BPL:BCCP complex has not yet been isolated. Moreover, detailed information is lacking on the composition, biosynthesis and role of fatty acids in hyperthermophilic organisms. We have cloned, overexpressed and purified recombinant BPL and the biotinyl domain of BCCP (BCCPDelta67) from the extreme hyperthermophile Aquifex aeolicus . In vitro assays have demonstrated that BPL catalyses biotinylation of lysine 117 on BCCPDelta67 at temperatures of up to 70 degreesC. Limited proteolysis of BPL with trypsin and chymotrypsin revealed a single protease-sensitive site located 44 residues from the N-terminus. This site is adjacent to the predicted substrate-binding site and proteolysis of BPL is significantly reduced in the presence of MgATP and biotin. Chemical crosslinking with 1-ethyl-3-(dimethylamino-propyl)-carbodiimide (EDC) allowed the isolation of a BPL:apo-BCCPDelta67 complex. Furthermore, this complex was also formed between BPL and a BCCPDelta67 mutant lacking the lysine residue (BCCPDelta67 K117L) however, complex formation was considerably reduced using holo-BCCPDelta67. These observations provide evidence that addition of the biotin prosthetic group reduces the ability of BCCPDelta67 to heterodimerize with BPL, and emphasizes that a network of interactions between residues on both proteins mediates protein recognition.