The formate channel FocA exports the products of mixed-acid fermentation

Abstract

Formate is a major metabolite in the anaerobic fermentation of glucose by many enterobacteria. It is translocated across cellular membranes by the pentameric ion channel/transporter FocA that, together with the nitrite channel NirC, forms the formate/nitrite transporter (FNT) family of membrane transport proteins. Here we have carried out an electrophysiological analysis of FocA from Salmonella typhimurium to characterize the channel properties and assess its specificity toward formate and other possible permeating ions. Single-channel currents for formate, hypophosphite and nitrite revealed two mechanistically distinct modes of gating that reflect different types of structural rearrangements in the transport channel of each FocA protomer. Moreover, FocA did not conduct cations or divalent anions, but the chloride anion was identified as further transported species, along with acetate, lactate and pyruvate. Formate, acetate and lactate are major end products of anaerobic mixed-acid fermentation, the pathway where FocA is predominantly required, so that this channel is ideally adapted to act as a multifunctional export protein to prevent their intracellular accumulation. Because of the high degree of conservation in the residues forming the transport channel among FNT family members, the flexibility in conducting multiple molecules is most likely a general feature of these proteins.electrophysiology | planar lipid bilayer | formate channel

Haemophilus ducreyi Seeks Alternative Carbon Sources and Adapts to Nutrient Stress and Anaerobiosis during Experimental Infection of Human Volunteers

“…Pyruvate generated from the decarboxylation of oxaloacetate subsequently is metabolized to acetylcoenzyme A, which ultimately is converted by pyruvate formate lyase into ATP and formate (36)(37)(38). The accumulation of formate results in significant reduction in intracellular pH, necessitating the excretion of formate out of the cell (39). H. ducreyi upregulated three genes encoding different components of citrate lyase (citC, citD, and citE) and a gene encoding the formate transporter (focA) in vivo (Table 3).…”

A lactate and formate transporter in the intraerythrocytic malaria parasite, Plasmodium falciparum

“…Formate is also present in significant quantities in asexual-stage P. falciparum parasites 19 . A recent study has highlighted the ability of a formate-nitrite transporter family member (FocA) from the enterobacterium Salmonella typhimurium to facilitate the export of the major end-products of anaerobic mixed-acid fermentation 11 . Such a role has not previously been demonstrated for eukaryote formate-nitrite transporter family members.…”

“…This protein falls within the microbial formate-nitrite family of transporters (FNT; TC 1.A.16), at least one other member of which has been shown to transport lactate and other products of anaerobic fermentation 11,12 .…”

Re-analysis of omics data indicates Smithella may degrade alkanes by addition to fumarate under methanogenic conditions

“…Genes for b-oxidation of fatty acids were also highly transcribed during growth on hexadecane (Embree et al, 2013), consistent with utilization of n-hexadecane via a fumarate activation pathway. Expression of putative PFL genes on contigs AWGX01000042, À 531 and À 777 (Supplementary Figure S1) during growth on hexadecane likely reflects conversion of pyruvate to acetyl-coA and formate (Lu et al, 2012), a process common in methanogenic substrate degradation.…”