Structural basis of 7SK RNA 5′ γ-phosphate methylation and retention by MePCE

Authors

• Yuan Yang, UCLA
• Catherine D. Eichhorn, UCLAFollow
• Yaqiang Wang, UCLA
• Duilio Cascio, UCLA
• Juli Feigon, UCLAFollow

Document Type

Article

Date of this Version

2019

Citation

Nat Chem Biol. 2019 February ; 15(2): 132–140

Comments

HHS Public Access

doi:10.1038/s41589-018-0188-z

Abstract

Among RNA 5′-cap structures, γ-phosphate monomethylation is unique to a small subset of noncoding RNAs, 7SK and U6 in humans. 7SK is capped by methylphosphate capping enzyme (MePCE), which has a second non-enzymatic role as a core component of the 7SK RNP that is an essential regulator of RNA transcription. We report 2.0 and 2.1 Å X-ray crystal structures of human MePCE methyltransferase domain bound to S-adenosylhomocysteine (SAH) and uncapped or capped 7SK substrates, respectively. 7SK recognition is achieved by protein contacts to a 5′ hairpin-single-stranded RNA region, explaining MePCE specificity for 7SK and U6. The structures reveal SAH and product RNA in a near-transition state geometry. Surprisingly, binding experiments show that MePCE has higher affinity for capped vs uncapped 7SK, with kinetic data supporting a slow product release model. This work reveals the molecular mechanism of methyl transfer and 7SK retention by MePCE for subsequent assembly of 7SK RNP.