Biotin protein ligase (BPL) is an enzyme mediating biotinylation of a specific lysine residue of the carboxyl carrier protein (BCCP) of biotin-dependent enzymes. We recently found that the substrate specificity of BPL from archaeon Sulfolobus tokodaii is totally different from those of many other organisms, in reflection of a difference in the local sequence of BCCP surrounding the canonical lysine residue. There is a conserved glycine residue in the biotin-binding site of Escherichia coli BPL, but this residue is replaced with alanine in S. tokodaii BPL. To test the notion that this substitution dictates the Substrate specificity of the latter enzyme, this residue, Ala-43, was converted to glycine. The K values of the resulting mutant, A43G, for Substrates, were smaller than those of the wild type, suggesting that the residue in position 43 of BPL plays an important role in substrate binding. (c) 2006 Elsevier Inc. All rights reserved.