The structures of the immunity proteins, Rap1a , responsible for the inhibition and neutralization of Ssp1 endopeptidase, revealed two distinct folds. The structure of the Ssp1-Rap1a complex revealed a tightly bound heteromeric assembly with two effector molecules flanking a Rap1a dimer. The Rap1a subunit displays a compact globular structure constructed from five alpha-helices that assemble to form the highly stable symmetric dimer [1].

1: Structural basis for type VI secreted peptidoglycan DL-endopeptidase function, specificity and neutralization in Serratia marcescens. Srikannathasan V, English G, Bui NK, Trunk K, O'Rourke PE, Rao VA, Vollmer W, Coulthurst SJ, Hunter WN; Acta Crystallogr D Biol Crystallogr. 2013;69:2468-2482. PMID:24311588