Fig 5. Model of the secretory machinery.

The SecYEG translocon (in green) is comprised SecYEG molecules which are engaged in cycles of association and dissociation with the SecY molecules on the membrane (a). SecA (shown in blue) delivers the pre-protein (shown in red) destined for export across the membrane to the translocon, where multiple molecules of SecA associate and accumulate during translocation. This association is dynamic because molecules of SecA exchange with the surrounding free molecules on the membrane (b) and in the cytoplasm (c). The SecYEG heterotrimer moves between neighboring translocons only when complexed with other proteins, such as SecA (d). A single SecA molecule can sequentially engage with neighboring SecYEG complexes (e).