9.A.4 The YggT or Fanciful K+ Uptake-B (FkuB; YggT) Family
Nakamura et al. (1996) identified a DNA fragment from Vibrio alginolyticus which complemented the growth defect of a K+ uptake deficient mutant of E. coli TK420. Two of the residing genes on this fragment were reported to be required for growth in synthetic medium containing 10 mM K+ and 100 mM Na+. K+ uptake was restored by introducing these genes.
Homologues of FkuB (most of 180-200 aas or of 80-100 aas) include the E. coli YggT protein. The larger members of this family contain a repeat found in many integral membrane proteins; the smaller members have just one.
In the E. coli YggT protein (188 aas), residues 1-91 are repeated (92-182). Half sized homologues (e.g., the YggT homologues of Geobacter uraniumreducens; 105 aas; gi#88936025, and Bacillus anthracis; YlmG; 87 aas; gi#49180675, both have just 2 TMSs). Other homologues are much larger with more than four TMSs (e.g., from Wolbachia; 992 aas; 6-8 putative TMSs with a large N-terminal hydrophilic domain and a smaller C-terminal hydrophilic domain) and the Type IV secretory pathway VirB6 component of Wolbachia (gi#58585051; 1040 aas; 8 putative TMSs with a long hydrophilic N-terminus of >600 aas).
The proposed transport function catalyzed by FkuB is:
K+ (out) → K+ (in)