Detection of cross-links between FtsH, YidC, HflK/C suggests a linked role for these proteins in quality control upon insertion of bacterial inner membrane proteins.

Little is known about the quality control of proteins upon integration in the inner membrane of Escherichia coli. Here, we demonstrate that YidC and FtsH are adjacent to a nascent, truncated membrane protein using in vitro photo cross-linking. YidC plays a critical but poorly understood role in the biogenesis of E. coli inner membrane proteins (IMPs). FtsH functions as a membrane chaperone and protease. Furthermore, we show that FtsH and its modulator proteins HflK and HflC copurify with tagged YidC and, vice versa, that YidC copurifies with tagged FtsH. These results suggest that FtsH and YidC have a linked role in the quality control of IMPs.

Commuting between Golgi cisternae--mind the GAP!

Intracellular transport has remained central to cell biology now for more than 40 years. Despite this, we still lack an overall mechanistic framework that describes transport in different parts of the cell. In the secretory pathway, basic questions, such as how biosynthetic cargo traverses the pathway, are still debated. Historically, emphasis was first put on interpreting function from morphology at the ultrastructural level revealing membrane structures such as the transitional ER, vesicular carriers, vesicular tubular clusters, Golgi cisternae, Golgi stacks and the Golgi ribbon. This emphasis on morphology later switched to biochemistry and yeast genetics yielding many of the key molecular players and their associated functions that we know today. More recently, microscopy studies of living cells incorporating biophysics and system analysis has proven useful and is often used to readdress earlier findings, sometimes with surprising outcomes.

Targeting and translocation of two lipoproteins in Escherichia coli via the SRP/Sec/YidC pathway.

In Escherichia coli, two main protein targeting pathways to the inner membrane exist: the SecB pathway for the essentially posttranslational targeting of secretory proteins and the SRP pathway for cotranslational targeting of inner membrane proteins (IMPs). At the inner membrane both pathways converge at the Sec translocase, which is capable of both linear transport into the periplasm and lateral transport into the lipid bilayer. The Sec-associated YidC appears to assist the lateral transport of IMPs from the Sec translocase into the lipid bilayer. It should be noted that targeting and translocation of only a handful of secretory proteins and IMPs have been studied. These model proteins do not include lipoproteins. Here, we have studied the targeting and translocation of two secretory lipoproteins, the murein lipoprotein and the bacteriocin release protein, using a combined in vivo and in vitro approach. The data indicate that both murein lipoprotein and bacteriocin release protein require the SRP pathway for efficient targeting to the Sec translocase. Furthermore, we show that YidC plays an important role in the targeting/translocation of both lipoproteins.

Versatility of inner membrane protein biogenesis in Escherichia coli.

To further our understanding of inner membrane protein (IMP) biogenesis in Escherichia coli, we have accomplished the widest in vivo IMP assembly screen so far. The biogenesis of a set of model IMPs covering most IMP structures possible has been studied in a variety of signal recognition particle (SRP), Sec and YidC mutant strains. We show that the assembly of the complete set of model IMPs is assisted (i.e. requires the aid of proteinaceous factors), and that the requirements for assembly of the model IMPs into the inner membrane differ significantly from each other. This indicates that IMP assembly is much more versatile than previously thought.

Assembly and overexpression of membrane proteins in Escherichia coli.

The bacterium Escherichia coli is one of the most popular model systems to study the assembly of membrane proteins of the so-called helix-bundle class. Here, based on this system, we review and discuss what is currently known about the assembly of these membrane proteins. In addition, we will briefly review and discuss how E. coli has been used as a vehicle for the overexpression of membrane proteins.

Targeting, insertion, and localization of Escherichia coli YidC.

YidC was recently shown to play an important role in the assembly of inner membrane proteins (IMPs) both in conjunction with and separate from the Sec-translocon. Little is known about the biogenesis and structural and functional properties of YidC, itself a polytopic IMP. Here we analyze the targeting and membrane integration of YidC using in vivo and in vitro approaches. The combined data indicate that YidC is targeted by the signal recognition particle and inserts at the SecAYEG-YidC translocon early during biogenesis, unlike its mitochondrial homologue Oxa1p. In addition, YidC is shown to be relatively abundant compared with other components involved in IMP assembly and is predominantly localized at the poles of the cell.