2.A.96 The Acetate Uptake Transporter (AceTr) Family
YaaH is an E. coli protein of 188aas with 6 putative TMSs. It has homologues in other bacteria, archaea, several fungi and protozoa. S. cerevisiae has 3 paralogues (YCR010c, YDR384c and YNR002c). The homlogue, AcpA, of Aspergillus nidulans, has been shown to be an acetate uptake porter in germinating conidia under conditions when the substrate is not protonated (Robellet et al., 2008). Another homologue of A. nidulans, AlcS, is much more distantly related but is of unknown transport function (Flipphi et al., 2006). In Saccharomyces cerevisiae, the orothologue of AcpA is Ady2 (50% identical). Evidence indicates that several other homologues are acetate porters (see proteins).
SatP (YaaH) of E. coli has been preliminarily identified as a succinate-acetate/proton symporter. Sun et al. 2018 reported the crystal structure of SatP at 2.8 Å resolution, which revealed a hexameric UreI-like channel structure. It has six TMSs surrounding the central channel pore in each protomer and three conserved hydrophobic residues, FLY, located in the middle of the TMS region for pore constriction. According to single-channel conductance recordings, performed with purified SatP reconstituted into lipid bilayers, three conserved polar residues in TMS1, facing the periplasmic side, are closely associated with acetate translocation activity (Sun et al. 2018). Wu et al. 2019 suggested that channel opening results from the repacking of key residues, such as Gln50 and Phe17, as well as the subsequent outward movement of all transmembrane helices. Their simulations suggested that acetate is always surrounded by several water molecules when passing through the channel. A high energy barrier of 15 kcal/mol was measured. This fact and the results observed for fungal homologues suggest that a carrier mechanism rather than a channel mechanism is operative.