For synthesis and localization of outer membrane (om) proteins three processes of basic interest are operating, using mechanisms which are still poorly understood: translocation across the plasma membrane, homing (sorting to om), and cross-regulation of their synthesis. Here we address the latter two mechanisms. We exclude the lipoproteins (see Chapter 14) and the TonB-dependent om receptors for which the sorting mechanism has not been investigated. Also, we did not attempt to review the relevant literature exhaustively; we generally restrict ourselves to those cases which have been studied most thoroughly: the porins OmpC and OmpF, the phosphoporin PhoE the maltodextrin porin LamB and the OmpA protein. All are synthesized as precursors with a signal sequence and use the Sec system for translocation. A monograph has been published on protein targeting, which includes eukaryotic systems and covers the literature up to 1988, another review discusses sorting of E. coli om proteins and covers the literature essentially also up to 1988. Figure 1 illustrates the topics in this chapter.
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