Increase in the size of the central tunnel.

The diameters correspond to circles defined by the Cα atoms of residues located at the tunnel's surface. In the case of TRAP, the small terminase (Bűttner et al., to be published) and the SAP-like pentraxin [25], which are composed of compact subunits containing single domains, the indicated diameter corresponds to residues 7, 95 and 185, respectively. In the case of the protective antigen [7], which contains multiple domains per subunit, the indicated diameter corresponds to residue 473 located in the oligomerization part of domain 2 where subunits interact with each other.