Mechanistic basis of choline import and lipid flipping involved in teichoic acid synthesis and modification.

Abstract

Teichoic acid decorated with phosphocholine molecules have fundamental roles in bacteria adhesion to host cells, immune evasion, and persistence. The biosynthesis of teichoic acids is a multistep process that involves several membrane proteins, including transporters of lipids and soluble precursors. Streptococcus pneumoniae is a prominent pathogen where phosphocholine decoration plays a fundamental role in virulence. This decoration occurs after choline uptake by LicB, and conversion to phosphocholine by LicA and LicC proteins. Teichoic acids decorated with phosphocholine, are then flipped across the plasma membrane, an essential step that regulates the population of the biopolymer at the cell wall. Cell wall exposed phosphocholine epitopes allow anchoring of choline-binding proteins, which contribute to adherence, colonization, and virulence. Here, we discuss cryo-EM and crystal structures of the membrane proteins involved in choline uptake and lipid flipping during teichoic acid synthesis in S. pneumoniae. We describe architectural and mechanistic elements essential to their activity, and together with in vitro and in vivo functional characterization, we study their role in the adaptation of S. pneumoniae to stress conditions. Our results provide previously unknown insights into the molecular mechanism of two transporters involved in bacterial pathogenesis and establish a basis for inhibiting the teichoic acid synthesis pathway in S. pneumoniae.