MicroRNA maturation is initiated by RNase III
DROSHA that cleaves the stem loop of primary
microRNA. DROSHA functions together with its
cofactor DGCR8 in a heterotrimeric complex known
as Microprocessor. Here, we report the X-ray structure
of DROSHA in complex with the C-terminal helix
of DGCR8. We find that DROSHA contains two
DGCR8-binding sites, one on each RNase III domain
(RIIID), which mediate the assembly of Microprocessor.
The overall structure of DROSHA is surprisingly
similar to that of Dicer despite no sequence
homology apart from the C-terminal part, suggesting
that DROSHA may have evolved from a Dicer homolog.
DROSHA exhibits unique features, including
non-canonical zinc-finger motifs, a long insertion in
the first RIIID, and the kinked link between Connector
helix and RIIID, which explains the 11-bp-measuring
‘‘ruler’’ activity of DROSHA. Our study implicates the
evolutionary origin of DROSHA and elucidates the
molecular basis of Microprocessor assembly and
primary microRNA processing. ª2016 Elsevier Inc.