Apparently weak and strong signal sequences are differentially targeted to the ER acording to a new paper in Cell.
(For more on how proteins are inserted into the ER click here.)
Preprolactin has a strong signal sequence and is inserted into the translocon even when the unfolded protein response (UPR) is activated. Prion protein, which has a weak signal sequence, is only inserted when UPR is inactive. Remember that under stress (high heat or other nasty conditions) cells rewire protein production and turn off the insertion of most proteins into the ER while upregulating the production and insertion of proteins required for dealing with ER stress such as chaperones (chaperones = proteins that help folding). This is mediated by UPR. If unfolded proteins accumulate within the ER these proteins are trashed and this process in mediated by ERAD (ER associated degradation, click here, here and here for more on ERAD and UPR).
So what's the mechanism of this differential insertion?
Well in a previous paper the Hegde group demonstrated that Prion protein gets inserted into the translocon with the help of the TRAP complex. TRAP is one of these translocon associated scaffolds that are thought to play some role in translocation although its role was not totally understood until this paper came out (another one of these translocon associated complexes is TRAM, it may help insert certain membrane associated proteins).
From the data presented in these two papers it seems like TRAP can help insert weak signal sequence containing proteins but is also inactivated by UPR. The authors also have some fancy experiments where they punch holes in the ER and demonstrate that this inhibits Prion protein insertion but not Preprolactin. This experiment suggests that factors inside the ER, like chaperones, may also be helping Prion protein to insert. This makes some sense in that fully occupied chaperones (i.e. too many proteins to fold within the ER) is a hallmark for UPR.
Some questions
Is this mechanism true for other proteins with weak and strong signal sequences? Is the strength of the signal sequence a good predictor of whether the protein needs chaperones to fold within the lumen of the ER? They could have addressed these issues with a couple of experiments. Are weakly folding proteins trapped in the translocon when the cell is undergoing stress? (David Ron's group has a Cell paper on this - but recent info has completely clouded this issue.)
Hope we get some answers soon.
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This is an interesting story. It reminds me of the cotransin story (Garrus et al, Nature, 2005) in that the inhibitor showed a distinct affect based on signal sequence strength.
BTM,
In the Ron paper, this drug is used to make some point about how p58 (a heat shock protein cofactor) promotes the destruction proteins that are stuck in the channel. With the drug, the substrate disappears - presumably because it is not inserted into the ER and p58 triggers its destruction. In p58 -/- cells the substrate becomes non-glycosylated in the presence of the drug (for non-ER people since glycosylation occurs inside the ER, an absence of glycosylation may indicate that the substrate never entered into the ER) and seems slightly less sensitive to the drug. However since p58 is likely to be luminal - I'm not sure how to interpret the results.
Not sure how that is relevant to the cotransin story but it is interesting to have a compound that will allow for more testing of some of these models. Someone really should get a co-crystal structure. Too bad it is really hard to make.
Cotransin is really hard to make. But anyone who wants to try for a co-crystal structure is welcome to, you only need to write us for the compound. And these days we can supply more potent derivatives.