The Sec system transports proteins before they acquire stable tertiary structure. YidC can also function independently as an insertase for proteins with short transmembrane sequences. YidC functions in concert with the SRP-Sec pathway in insertion of cytoplasmic membrane proteins and assembly of multimeric membrane proteins. Membrane proteins integral to the cytoplasmic membrane are inserted co-translationally while undergoing elongation on the ribosome and are recruited to the Sec system by the signal recognition particle (SRP). Lipoproteins arrive in the periplasm and are sorted either to the cytoplasmic membrane or to the outer membrane by the Lol transport system. A multiprotein complex, the β barrel assembly machine, BAM, facilitates transfer and insertion of the β-barrel proteins into the outer membrane. Proteins destined for the outer membrane engage several periplasmic chaperones. The periplasm is the final destination for some proteins but for others it is only the initial step. This chapter focuses on the general secretory system, the Sec system, which handles the great majority of proteins that are exported from the site of synthesis of all bacterial proteins, the cytoplasm, into the cytoplasmic membrane, the outer membrane or the aqueous space between, the periplasm. coli has eight different systems for export of protein.
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