Signal sequence-triage is activated by translocon obstruction sensed by an ER stress sensor IRE1

Secretory pathway proteins are cotranslationally translocated into the endoplasmic reticulum (ER) of metazoan cells through the protein channel, translocon. Given that there are far fewer translocons than ribosomes in a cell, it is essential that secretory protein-translating ribosomes only occupy translocons transiently. Therefore, if translocons are obstructed by ribosomes stalled or slowed in translational elongation, it possibly results in deleterious consequences to cellular function. Hence, we investigated how translocon clogging by stalled ribosomes affects mammalian cells. First, we constructed ER-destined translational arrest proteins (ER-TAP) as an artificial protein that clogged the translocon in the ER membrane. Here, we show that the translocon clogging by ER-TAP expression activates triage of signal sequences (SS) in which secretory pathway proteins harboring highly efficient SS are preferentially translocated into the ER lumen. Interestingly, the translocon obstructed status specifically activates inositol requiring enzyme 1 alpha (IRE1 alpha) but not protein kinase R-like ER kinase (PERK). Given that the IRE1 alpha-XBP1 pathway mainly induces the translocon components, our discovery implies that lowered availability of translocon activates IRE1 alpha, which induces translocon itself. This results in rebalance between protein influx into the ER and the cellular translocation capacity.