N-linked glycosylation is a protein modification needed for protein folding in the endoplasmic reticulum (ER). If unfolded proteins accumulate in the ER,then the `unfolded protein response' (UPR) is triggered, increasing folding rates and reducing translation rates. On , Mattias Mannervik and colleagues describe the first embryonic patterning defects known to be caused by an inappropriate UPR. In their screen for maternal factors involved in embryonic patterning, they discovered a mutant -wollknäuel (wol) - that has reduced Dpp signalling,posterior segmentation defects due to a lack of the transcription factor Caudal, and defects in germband elongation and retraction. wolencodes ALG5, a UDP-glucose:dolichyl-phosphate glucosyltransferase involved in N-linked glycosylation, and its mutation causes the accumulation of unglycosylated proteins and triggers the UPR. One component of the UPR is the phosphorylation of the translation initiation factor eIF2α, which attenuates protein translation. These findings suggest that some mRNAs, such as caudal, are particularly sensitive to eIF2α phosphorylation,resulting in the wol patterning defects.
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