Structure of the aliphatic sulfonate‐binding protein SsuA from Escherichia coli

Sulfur is an essential component for the biosynthesis of the sulfur‐containing amino acids l ‐methionine and l ‐cysteine. Under sulfur‐starvation conditions, bacteria are capable of scavenging sulfur from sulfur‐containing compounds and transporting it across membranes. Here, the crystal structure of the periplasmic aliphatic sulfonate‐binding protein SsuA from Escherichia coli is reported at 1.75 Å resolution in the substrate‐free state. The overall structure of SsuA resembles the structures of other periplasmic binding proteins and contains two globular domains that form a cleft. Comparison with other periplasmic binding proteins revealed that one of the domains has been displaced by a rigid movement of 17°. Interestingly, the tight crystal packing appears to be mediated by a 13‐amino‐acid tail from the cloning that folds within the cleft of the next monomer.