Fig. 1.

Cation dependence of the V.SecDF paralogs expressed in E. coli: V.SecDF1 requires Na⁺. (A) Gene organization of the secDF loci in E. coli and V. alginolyticus. ORFs are shown by arrows. (B) Protein export-enhancing activities of V.SecDF1 and V.SecDF2 assessed using the E. coli secD1(Cs) mutant. E. coli secD1 cells expressing the indicated SecDF protein were grown in M63 medium supplemented with 300 mM Na⁺ or K⁺, as indicated, at 37 °C until an early log phase, shifted to 20 °C, and cultivated for additional 30 min. Cells were pulse-labeled with [³⁵S]methionine for 1 min. Immunoprecipitated precursor (p) and mature (m) forms of labeled MBP and OmpA were separated by 10% Laemmli gel and detected by phosphorimaging (Upper). Export efficiencies (proportions of mature form) of MBP (gray) and OmpA (black) are shown with SD (n ≥ 3) (Lower). See also Fig. S1.