Mechanism of precise intracellular targeting of spontaneously inserting tail-anchored proteins

Tail-anchored (TA) proteins are membrane proteins that are targeted to their destination by unique post-translational pathways. Cytochrome b5 is a spontaneously inserted TA protein, of which two forms are known: one targeting the ER (b5-ER) and other targeting the MOM (b5-RR). Microinjection of the recombinant proteins into cells results in precise targeting of each of the two proteins, indicating that the targeting information is present in the protein. Using digitonin semi-permeabilized cells and in the presence of rabbit reticulocyte lysate (RRL), we have obtained faithful targeting for both forms of cytochrome b5 that approaches the in cellula situation. In contrast, in the absence of cytosol both forms target the mitochondria. Attempting to find the chaperone responsible for the ER targeting, we observed that both TCR40 and Snd2 pathways play only a modest role, suggesting the contribution of redundant pathways to the biogenesis of b5. Recently, we found that Eyerestatin I, a p97 inhibitor, strongly inhibits the glycosylation of b5-ER. The effect seems to be specific for spontaneously inserted tail-anchored proteins, but the exact mechanism is still under investigation.