The Membrane Proteins SiaQ and SiaM Form an Essential Stoichiometric Complex in the Sialic Acid Tripartite ATP-independent Periplasmic (TRAP) Transporter SiaPQM (VC1777-1779) from Vibrio cholerae

Tripartite ATP-independent periplasmic (TRAP) transporters are widespread in bacteria but poorly characterized. They contain three subunits, a small membrane protein, a large membrane protein, and a substrate-binding protein (SBP). Although the function of the SBP is well established, the membrane components have only been studied in detail for the sialic acid TRAP transporter SiaPQM from Haemophilus influenzae, where the membrane proteins are genetically fused. Herein, we report the first in vitro characterization of a truly tripartite TRAP transporter, the SiaPQM system (VC1777-1779) from the human pathogen Vibrio cholerae. The active reconstituted transporter catalyzes unidirectional Na+-dependent sialic acid uptake having similar biochemical features to the orthologous system in H. influenzae. However, using this tripartite transporter, we demonstrate the tight association of the small, SiaQ, and large, SiaM, membrane proteins that form a 1:1 complex. Using reconstituted proteoliposomes containing particular combinations of the three subunits, we demonstrate biochemically that all three subunits are likely to be essential to form a functional TRAP transporter.