(a) Coomassie-stained gel of protein elution from NiNTA resin shows that double-hexahistidine (12HIS)-tagged SpaO (‘FL,’ full length) and SpaO(140–297, Val203GTT) are each sufficient to co-affinity purify InvC–OrgB when co-expressed in Escherichia coli. Note that OrgB is necessary for ternary complex formation. Asterisk denotes nonspecific co-purifying E. coli proteins, likely chaperones. SpaOc indicates the cryptically expressed SPOA2-containing carboxy-terminal fragment. Data shown are representative of three experiments. (b) Ribbon diagram of the SpaO–OrgB crystal structure. For simplicity, the T4 lysozyme crystallization chaperone has been omitted and only one of the two constituent complexes from the crystallographic asymmetric unit is shown. The amino and carboxy termini of the OrgB APAR are denoted as ‘N’ and ‘C,’ respectively. (c) Surface representation of the complex in b. The OrgB APAR (grey mesh) contacts both SpaO SPOA1 (cyan) and SPOA2 (green).