Structural and mutational analysis of the ribosome-arresting human XBP1u

XBP1u, a central component of the unfolded protein response (UPR), is a mammalian protein containing a functionally critical translational arrest peptide (AP). Here, we present a 3 A cryo-EM structure of the stalled human XBP1u AP. It forms a unique turn in the upper part of the ribosomal exit tunnel and causes a subtle distortion of the peptidyl transferase center, explaining the temporary translational arrest induced by XBP1u. During ribosomal pausing the hydrophobic region 2 (HR2) of XBP1u is recognized by SRP, but fails to efficiently gate the Sec61 translocon. An exhaustive mutagenesis scan of the XBP1u AP revealed that only 10 out of 21 mutagenized positions in the XBP1u AP are optimal with respect to translational arrest activity. Thus, XBP1u has evolved to induce an intermediate level of translational arrest, allowing efficient targeting by SRP without activating the Sec61 channel and thereby serving its central function in the UPR.