supported by Wellcome (102535/Z/13/Z). P.C. was supported by the Deutsche
Forschungsgemeinschaft (SFB860, SPP1935, EXC 2067/1-390729940), the European Research
Council (advanced investigator grant TRANSREGULON, grant agreement no. 693023) and the
Volkswagen Foundation.
Author contributions H.W. carried out all experiments and data analysis except mass
spectrometry analysis. C.D. assisted with cryo-EM data collection. A.C.M.C. contributed to
developing the purification protocol and assisted with model building. A.S. and H.U. carried
out mass spectrometry analysis. P.C. supervised research. H.W. and P.C. interpreted the data
and wrote the manuscript, with input from all authors.
Competing interests The authors declare no competing interests.Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41586-020-
1933-5.
Correspondence and requests for materials should be addressed to P.C.
Peer review information Nature thanks Steve Hahn and the other, anonymous, reviewer(s) for
their contribution to the peer review of this work.
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