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It is well known that the endoplasmic reticulum (ER) is capable of expanding its surface area in response both to cargo load and to increased expression of resident membrane proteins. Although the response to increased cargo load, known as the unfolded protein response (UPR), is well characterized, the mechanism of the response to membrane protein load has been unclear. As a model system to investigate this phenomenon, we have used a HeLa-TetOff cell line inducibly expressing a tail-anchored construct consisting of an N-terminal cytosolic GFP moiety anchored to the ER membrane by the tail of cytochrome b5 [GFP-b(5)tail]. After removal of doxycycline, GFP-b(5)tail is expressed at moderate levels (1-2% of total ER protein) that, nevertheless, induce ER proliferation, as assessed both by EM and by a three- to fourfold increase in phosphatidylcholine synthesis. We investigated possible participation of each of the three arms of the UPR and found that only the activating transcription factor 6 (ATF6) arm was selectively activated after induction of GFP-b(5)tail expression; peak ATF6a activation preceded the increase in phosphatidylcholine synthesis. Surprisingly, up-regulation of known ATF6 target genes was not observed under these conditions. Silencing of ATF6a abolished the ER proliferation response, whereas knockdown of Ire1 was without effect. Because GFP-b(5)tail lacks a luminal domain, the response we observe is unlikely to originate from the ER lumen. Instead, we propose that a sensing mechanism operates within the lipid bilayer to trigger the selective activation of ATF6. [ABSTRACT FROM AUTHOR]

C-tail-anchored (TA) proteins constitute a heterogeneous group of membrane proteins that are inserted into membranes by unique post-translational mechanisms and that play key roles within cells. During recent years, bioinformatic screens on eukaryotic genomes have helped to obtain comprehensive pictures of the number, intracellular distribution and functions of TA proteins, but similar screens had not yet been carried out on prokaryotic cells. Here, we report the results of a bioinformatic screen of the genomes of two bacteria and one archeon. We find that all three of these prokaryotes contain TA proteins in proportions approaching those found in eukaryotic cells, indicating that this protein group is present in all three domains of life. Although some of our hits correspond to proteins of unknown function, others are enzymes with hydrophobic substrates or have functions carried out at the inner face of the cytoplasmic membrane. To generate hypotheses on the insertion mechanisms of prokaryotic TA proteins, we compared the sequences of the prokaryotic and eukaryotic versions of Asna1/Trc40/GET3, a cytosolic ATPase that plays a key role in TA protein post-translational delivery to membranes in eukaryotic cells. We found that hydrophobic residues involved in TA binding by the eukaryotic chaperone (Mateja et al., Nature 2009;461:361–366) are generally replaced with equally hydrophobic amino acids in the archeal homologue (ArsA), whereas this is not the case for the bacterial protein. Thus, eukaryotes may have inherited the GET3 targeting pathway from our archeal ancestor, while the bacterial homologue may be exclusively dedicated to heavy metal resistance. [ABSTRACT FROM AUTHOR]

A large class of proteins with cytosolic functional domains is anchored to selected intracellular membranes by a single hydrophobic segment close to the C-terminus. Although such tail-anchored (TA) proteins are numerous, diverse, and functionally important, the mechanism of their transmembrane insertion and the basis of their membrane selectivity remain unclear. To address this problem, we have developed a highly specific, sensitive, and quantitative in vitro assay for the proper membrane-spanning topology of a model TA protein, cytochrome b5 (b5). Selective depletion from membranes of components involved in cotranslational protein translocation had no effect on either the efficiency or topology of b5 insertion. Indeed, the kinetics of transmembrane insertion into protein-free phospholipid vesicles was the same as for native ER microsomes. Remarkably, loading of either liposomes or microsomes with cholesterol to levels found in other membranes of the secretory pathway sharply and reversibly inhibited b5 transmembrane insertion. These results identify the minimal requirements for transmembrane topogenesis of a TA protein and suggest that selectivity among various intracellular compartments can be imparted by differences in their lipid composition. [ABSTRACT FROM AUTHOR]