Structure, function, evolution and application of bacterial Pnu-type vitamin transporters

Abstract

Many bacteria can take up vitamins from the environment via specific transport machineries. Uptake is essential for organisms that lack complete vitamin biosynthesis pathways, but even in the presence of biosynthesis routes uptake is likely preferred, because it is energetically less costly. Pnu transporters represent a class of membrane transporters for a diverse set of B-type vitamins. They were identified 30 years ago and catalyze transport by the mechanism of facilitated diffusion, without direct coupling to ATP hydrolysis or transport of coupling ions. Instead, directionality is achieved by metabolic trapping, in which the vitamin substrate is converted into a derivative that cannot be transported, for instance by phosphorylation. The recent crystal structure of the nicotinamide riboside transporter PnuC has provided the first insights in substrate recognition and selectivity. Here, we will summarize the current knowledge about the function, structure, and evolution of Pnu transporters. Additionally, we will highlight their role for potential biotechnological and pharmaceutical applications.

Structure and function of SemiSWEET and SWEET sugar transporters

“…Although heptahelical vitamin transporter PnuC and G protein-coupled receptors (GPCRs) were proposed to be related to SWEETs based on remote sequence pattern similarity and positions of the TM helices [24][25][26], their topologies differ from that of the SemiSWEET dimer. TMs of GPCR and PnuC core are connected sequentially, with each TM interacting with the next one in the sequence, which is in contrast to the 1-3-2 connectivity in THB of SemiSWEET.…”