The nuclear phosphoprotein La was identified as an autoantigen in patients with systemic lupus erythematosus and Sjogren's syndrome. La binds to and protects the UUUOH 3′ terminii of nascent RNA polymerase III transcripts from exonuclease digestion. We report the 1.85 Å crystal structure of the N-terminal domain of human La, consisting of La and RRM1 motifs, bound to r(U1-G2-C3-U4-G5-U6-U7-U8-U9OH). The U7-U8-U9 OH 3′ end, in a splayed-apart orientation, is sequestered within a basic and aromatic amino acid-lined cleft between the La and RRM1 motifs. The specificity-determining U8 residue bridges both motifs, in part through unprecedented targeting of the β sheet edge, rather than the anticipated face, of the RRM1 motif. Our structural observations, supported by mutation studies of both La and RNA components, illustrate the principles behind RNA sequestration by a rheumatic disease autoantigen, whereby the UUU OH 3′ ends of nascent RNA transcripts are protected during downstream processing and maturation events.
Bibliographical noteFunding Information:
This research was supported by funds from the Abby Rockefeller Mauze Trust and the Dewitt Wallace and Maloris Foundations to D.J.P. We thank Yuan Cheng for assistance with graphics. Drs. Elena Mossesova and Christopher Lima kindly provided the modified pET28b vector that adds an Ulp1 protease-cleavable His 6 -Smt3-tag at the N terminus. We thank personnel at synchrotron beam lines X25 at the National Synchrotron Light Source at the Brookhaven National Laboratory and 19BM at the Advanced Photon Source (APS) at Argonne National Laboratory for their assistance. Use of the APS beamline was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science. D.J.P. is a member of the New York Structural Biology Center, supported in part by National Institutes of Health funds.