Crystal structure of the sugar binding domain of the archaeal transcriptional regulator TrmB

TrmB is an alpha-glucoside-sensing transcriptional regulator controlling two operons encoding maltose/trehalose and maltodextrin ABC transporters of Pyrococcus furiosus. The crystal structure of an N-terminal truncated derivative of TrmB (amino acids 2-109 deleted; TrmB(Delta 2-109)) was solved at 1.5 angstrom resolution. This protein has lost its DNA binding domain but has retained its sugar recognition site. The structure represents a novel sugar-binding fold. TrmB(Delta 2-109) bound maltose, glucose, sucrose, and maltotriose, exhibiting K-d values of 6.8, 25, 34, and 160 mu M, respectively. TrmB(Delta 2-109) behaved as a monomer in dilute buffer solution in contrast to the full-length protein, which is a dimer. Co-crystallization with bound maltose identified a binding site involving seven amino acid residues: Ser(229), Asn(305), Gly(320), Met(321), Val(324), Ile(325), and Glu(326). Six of these residues interact with the nonreducing glucosyl residue of maltose. The nonreducing glucosyl residue is shared by all substrates bound to TrmB, suggesting it as a common recognition motif.