C. Co-immunoprecipitation assay. The solubilized lysates from 2×1010 E. coli K-12 W3110 cells co-producing the indicated FLAG-tagged TssMP variants (exported in the periplasm) and VSV-G-tagged MltE protein (Total, T) were subjected to immune precipitation on anti-FLAG-coupled agarose beads. The immunoprecipitated material (IP) was subjected to 12.5%-acrylamide SDS-PAGE and immunodetected with anti-FLAG (upper panel, TssM domains) and anti-VSV-G (lower panel, MltE) antibodies. Molecular weight markers (in kDa) are indicated on the left. The experiment was performed in triplicate and a representative result is shown. 