Nicotinamide ribonucleoside uptake transporters

PnuC of Salmonella typhimurium and Haemophilus influenzae are believed to function cooperatively with NadR homologues, multifunctional proteins that together with PnuC, participate in NR phosphorylation, transport and transcriptional regulation.[3][4][5] NadR, a cytoplasmic protein that is partly membrane associated, contains one well conserved and one poorly conserved mononucleotide-binding consensus sequence (G-X4 GKS). It drives transport and may render transport responsive to internal pyridine nucleotide levels. While its N-terminal half functions as a repressor, its C-terminal half functions as an NR kinase in a putative group translocation process.[6]

The components of the H. influenzae pathway necessary for NAD⁺, NMN, and NR uptake have been determined. Merdanovic et al. characterized two enzymes, an outer membrane nucleotide phosphatase, and an NAD⁺ nucleotidase (NadN) located in the periplasm.[9][10][11] They showed that NAD⁺ and NMN cross the outer membrane mainly via the OmpP2 porin.[12]

Only NR can be utilized by the PnuC transport system located in the inner membrane.[13][14] The pnuC gene product is the protein that is responsible for the main flow of the NR substrate into the cytoplasm. The study of Merdanovic et al. suggests that the RNK activity of NadR determines NR transport and is negatively regulated by cytoplasmic NAD⁺ feedback inhibition. Therefore, NR uptake is under NadR feedback control.[4]

ATP, not the proton motive force, appears to be required for NR uptake. Thus, the driving force for NR uptake via PnuC is NR phosphorylation by NadR. A concerted group translocation mechanism can be considered whereby NadR facilitates the dissociation of NR from PnuC by phosphorylating it to NMN, thus preventing efflux of NR.

PnuC of Salmonella typhimurium and Haemophilus influenzae are integral membrane proteins, 239 and 226 amino acyl residues (aas) in length, respectively, with 7 putative transmembrane α-helical segments.

The structure of NadR has been determined.[15] Mutations in the nadR gene which interfere with NR uptake occur in the C-terminal part of NadR.[16] A helix-turn-helix DNA binding domain present in NadR of S. enterica serovar Typhimurium [16] could not be found in the NadR homologue of H. influenzae. Therefore, it was proposed that in H. influenzae NadR has no regulatory function at the transcriptional level.[7] The structures of the human NR kinase 1 (2QL6_P) with nucleotide and nucleoside substrates bound have been solved.[17] It is structurally similar to Rossmann fold metabolite kinases.

PnuC has been shown to resemble SWEET porters in overall fold,[18] supporting the conclusion that these two families are members of the TOG superfamily.[1]