9.A.28 The Ethanolamine Facilitator (EAF) Family
Penrod et al. (2004) have provided convincing evidence that the EutH protein of Salmonella enterica mediates the pH-sensitive facilitated diffusion of uncharged ethanolamine (EA) in Salmonella enterica. The protein is required for EA entry into cells at a low pH or when the EA concentration is reduced to less than 25 μM. EutH+ cells grow normally with uncharged EA concentrations above 3 μM, close to the Km (9 μM) of the first degradative enzyme, EA·NH4 ligase. EutH also contributes to the toxicity of EA as expected for a transporter. It has 8-11 TMSs. It is not known if it is a channel or carrier. Homologues are found in Gram-positive and Gram-negative bacteria. It may have distant archaeal homologues (i.e., Pyrococcus abyssi, an 11 TMS protein, D75104).
Ceramide is produced by the condensation of a long chain base with a very long chain fatty acid. In Saccharomyces cerevisiae, one of the two major long chain bases is called phytosphingosine (PHS). PHS has been shown to cause toxicity in tryptophan auxotrophic strains of yeast because this bioactive ceramide precursor causes diversion of the high affinity tryptophan permease Tat2 (TC# 2.A.3.10.8) to the vacuole rather than the plasma membrane. Loss of the integral membrane protein Rsb1 increased PHS sensitivity, which was suggested to be due to this protein acting as an ATP-dependent long chain base efflux protein. Loss of the genes encoding the ATP-binding cassette transporter proteins Pdr5 and Yor1 elevate PHS tolerance. This increased resistance was suggested to be due to increased expression of RSB1.
Johnson et al. (2010) provided an alternative view of PHS resistance influenced by Rsb1 and Pdr5/Yor1. Rsb1 has a seven-transmembrane domain topology consistent with that of a regulatory protein. A Δrsb1 cell does not exhibit higher internal levels of PHS compared with isogenic wild-type cells. However, tryptophan transport is increased in Δpdr5 yor1 strains and reduced in Δrsb1 cells. Localization and vacuolar degradation of Tat2 are affected in these genetic backgrounds. Finally, internalization of FM4-64 dye suggests that loss of Pdr5 and Yor1 slows normal endocytic rates. Rsb1, Pdr5, and Yor1 thus may regulate the endocytosis of Tat2 and other membrane transporter proteins (Johnson et al., 2010).
The reaction catalyzed by EutH is:
Uncharged ethanolamine (out) ⇌ uncharged ethanolamine (in)