Alanine point mutants generated within α-helix 2 of VipA. Shown is the amino acid sequence of residues 103–127 predicted to form α-helix 2 within VipA of V. cholerae strain A1552 as well as the homologous region within IglA of F. tularensis LVS, according to Psipred (http://bioinf.cs.ucl.ac.uk/psipred/). A deletion within the first part (Δ104-113) of the α-helix abolishes VipA’s ability to bind to VipB in both B2H and Y2H systems (−), while deletions within the second part (Δ114-123) results in a VipA variant that retains VipB binding in the Y2H system, but not in the B2H system (+/−). Amino acids that were replaced with alanine in VipA are indicated by closed triangles. Residues in F. tularensis IglA that previously were mutated and shown to contribute to efficient IglB binding are indicated also by closed triangles .