(a) ErpC, ErpP and ErpA possess a common architecture. Sequence alignments of ErpC with ErpP and ErpA show high levels of conservation within the secondary-structure elements of ErpC (shown above), suggesting that all three proteins have the same fold. Sequence differences occur mainly in the loop regions between β-strands, suggesting that these regions may have evolved to bind specific complement proteins. Loop regions observed in the ErpC crystal structure are highlighted by a continuous green line. Those which were not observed are shown by a dotted green line. (b) Mapping of sequence similarity onto the structure of ErpC. Sequence differences between ErpC, ErpP and ErpA highlighted in (a) are coloured in blue on the surface representation of ErpC.