Western blot. For western blots, proteins from dissociated ectosymbionts were
separated by reduced SDS–polyacrylamide gel electrophoresis (PAGE) on NuPAGE
4–12% Bis-Tris pre-cast gels (Invitrogen), followed by transfer to Hybond ECL
nitrocellulose membranes (Amersham Biosciences). Membranes were blocked for
45 min in PBS containing 5% (wt/vol) nonfat milk (PBSM) at room temperature and
incubated overnight at 4 °C with either a rabbit polyclonal anti-E. coli FtsZ
antibody34 (1:400 dilution) or a sheep polyclonal anti-E. coli pili antibody (ab35292,
Abcam) raised against E. coli K88 fimbrial protein AB/FaeG (http://www.uniprot.
org/uniprot/P02970; 1:1,000 dilution) in PBSM. For the negative control, the
primary antibody was omitted at this step. After three washing steps in PBSM to
remove unbound antibody, the blots were incubated for 1 h at room temperature
with a horseradish peroxidase-conjugated anti-rabbit or anti-sheep secondary
antibody (1:5,000, Amersham Biosciences) in PBSM. Protein–antibody
complexes were visualized using ECL Plus detection reagents and films
(Amersham Biosciences).