Correction to: Nature Structural and Molecular Biology https://doi.org/10.1038/s41594-019-0369-9, published online 27 January 2020.

In the version of this article initially published, in Fig. 4a, the eye on the left was incorrectly labeled “Panel d” instead of “Panel c”. The eye on the right was incorrectly labeled “Panel c” instead of “Panel b”. The errors have been corrected in the HTML and PDF versions of the article.