Table 2. Unique features and possible functions for each subfamily of chaperones.

Characteristic features	FGL	FGS	FGA	Proposed functions of unique features (Mutations introduced in CfaA)
Average length of the F1–G1 loop	24 (long)	13 (short)	20 (long)
No. required hydrophobic residues in donor strand	4	3	4	Binding cognate pilin subunit (L114A/V116A/I118A/Y120A)
N-terminal extension (12–14 residues)	Yes	No	No
Disulfide bond between F1–G1 strands	Yes	No	No
D1′ insertion	No	No	Yes	Pilin or usher interaction (T44A/E45A/E46A)
C2–D2′ insertion	No	No	Yes	Pilin or usher interaction (K164-N171; 8×A)
Disulfide bond in C2–D2′ loop	No	No	Yes	Stabilizing C2–D2′ insertion (C163S/C172S)
Length of peptide linker between two domains	14 (long)	14 (long)	7 (short)
Residue for subunit anchoring	R20a	R8b	R154	Pilin subunit interaction (K9A/E86A/R125A/R154A)

^aR20 is for Caf1M of Y. pestis.

^bR8 is for PapD of P pili.