Structure and role of coupling proteins in conjugal DNA transfer

Type IV secretory systems are transmembrane bacterial multiprotein complexes. They are pivotal for conjugation, bacterial-induced plant tumour formation, toxin secretion and mammalian pathogen intracellular activity. These systems are involved in the spread of antibiotic resistance genes among bacteria by enabling conjugative DNA transfer. When such translocons transport DNA, they require the assistance of multimeric integral inner membrane proteins, the type IV coupling proteins. Its structural prototype is plasmid R388 TrwB protein, responsible for coupling the relaxosome with the DNA transport apparatus during bacterial conjugation. Its monomeric molecular structure is reminiscent of ring helicases and AAA ATPases. The quaternary structure is made up by six equivalent protomers featuring a flattened sphere resembling F1-ATPase, with a central channel traversing the particle, thus connecting cytoplasm and periplasm. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.This study was supported by grants PB98-1631, BIO2000-1659 and 2FD97-0518 from the Ministerio de Educación y Cultura and the Ministerio de Ciencia y Tecnología, Spain, by grants 1999SGR188 and 2001SGR346 and the Centre de Referència en Biotecnologia, both from the Generalitat de Catalunya, by the European Union (grants HPR1-CT-1999-00017 and ERBFMGCECT980134 to EMBL Outstation Hamburg and grants HPRI-CT-1999-00022 and ERBFMGECT980133 to EMBL Outstation Grenoble) and by the ESRF (all to F.X.G.R. and M.C.). The work of the F.C. laboratory was supported by grant PB98-1106 from DGICYT, Ministerio de Ciencia y Tecnología, SpainPeer Reviewe