An inner membrane platform in the type II secretion machinery of Gram-negative bacteria

The type II secretion machinery allows most Gram-negative bacteria to deliver virulence factors into their surroundings. We report that in Erwinia chrysanthemi, GspE (the putative NTPase), GspF, GspL and GspM constitute a complex in the inner membrane that is presumably used as a platform for assembling other parts of the secretion machinery. The GspE-GspF-GspL-GspM complex was demonstrated by two methods: (i) co-immunoprecipitation of GspE-GspF-GspL with antibodies raised against either GspE or GspF; (ii) interactions in the yeast two-hybrid system between GspF and GspE, GspF and GspL, GspL and GspM. GspL was found to have an essential role in complex formation. We propose a model in which the GspE-GspF-GspL-GspM proteins constitute a building block within the secretion machinery on top of which another building block, referred to as a pseudopilus, assembles. By analogy, we predict that a similar platform is required for the biogenesis of the type IV pilus.