1. Describe in detail how lipoproteins are secreted to the outer membrane. How is Sec system involved. What part do β-barrels play in the process? How does the process differ for lipoproteins attached to the periplasmic face of the inner membrane and the outer membrane (Figure 1).
2. Describe in detail the two hypothesis for LPS movement to the outer membrane.
3. Compare and contrast in detail secretion of outer membrane lipoproteins and LPS.
1. The protein is synthesized in the cytoplasm with a 25 amino acid N-terminal signal sequence which is recognized by SecA and SecB proteins. These two proteins deliver the protein to the SecYEG channel complex which translocates the protein across the inner membrane into the periplasmic space. When it does this, a signal peptidase cleaves the signal sequence off the protein. For lipoproteins, various lipid moieties are added which facilitate the lipoprotein being embedded in the inner membrane. For β-barrel proteins, a number of chaperone proteins such as SurA, Skp, and DegP prevent misfolding and clumping together. They transport the developing β-barrel protein to the outer membrane. A complex of proteins exist at the outer membrane which facilitates insertion. This is the YaeT complex which also contains four lipoproteins. This complex facilitates insertion of the new β-barrel lipoprotein into the outer membrane.
But let's go back to the lipoproteins which had the lipid moiety added and got imbedded in the IM. How do they get to the OM? There is a system which transports lipoproteins from ...