review
©2005 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION EMBO reports VOL 6 | NO 1 | 2005
review
RNA sequence- and shape-dependent recognition
by proteins in the ribonucleoprotein particle
Richard Stefl,Lenka Skrisovska & Frédéric H.-T.Allain+
Swiss Federal Institute of Technology Zürich,Zürich,Switzerland
At all stages of its life (from transcription to translation), an RNA
transcript interacts with many different RNA-binding proteins. The
composition of this supramolecular assembly, known as a ribonucleoprotein
particle, is diverse and highly dynamic. RNA-binding
proteins control the generation, maturation and lifespan of the RNA
transcript and thus regulate and influence the cellular function of
the encoded gene. Here, we review our current understanding of
protein–RNA recognition mediated by the two most abundant RNAbinding
domains (the RNA-recognition motif and the double-stranded
RNA-binding motif) plus the zinc-finger motif, the most abundant
nucleic-acid-binding domain. In addition, we discuss how not only
the sequence but also the shape of the RNA are recognized by these
three classes of RNA-binding protein.
Keywords: double-stranded RNA-binding motif; RNA-binding
proteins; RNA recognition; RNA-recognition motif; zinc-finger motif
EMBO reports (2005) 6, 33–38. doi:10.1038/sj.embor.7400325
Introduction
The association of RNA-binding proteins (RBPs) with RNA transcripts
begins during transcription. Some of these early-binding
RBPs remain bound to the RNA until it is degraded, whereas others
recognize and transiently bind to RNA at later stages for specific
processes such as splicing, processing, transport and localization
(Dreyfuss et al, 2002). The RBPs cover the RNA transcripts and control
their fate. Some RBPs function as RNA chaperones (Lorsch,
2002) by helping the RNA, which is initially single-stranded, to form
various secondary or tertiary structures. When folded, these structured
RNAs, together with specific RNA sequences, act as a signal
for other RBPs that mediate gene regulation. Here, we review our
current structural understanding of protein–RNA recognition mediated
by the two most abundant RNA-binding domains, the RNArecognition
motif (RRM) and the double-stranded RNA-binding
motif (dsRBM), and by the most abundant nucleic-acid-binding
motif, the CCHH-type zinc-finger domain. We discuss how these
three small domains recognize RNA: some bind single-stranded
RNA by direct readout of the primary sequence, whereas others
recognize primarily the shape of the RNA or both the sequence and
the shape. Other types of RNA-binding domains, such as the
K-homology (KH) domain or the oligonucleotide/oligosaccharidebinding
(OB) fold, have recently been reviewed and are not
discussed here (Messias & Sattler, 2004).
RNA shape-dependent recognition by double-stranded RBM
The dsRBM is a 70–75 amino-acid domain with a conserved αβββα
protein topology in which the two α-helices are packed along one
face of a three-stranded anti-parallel β-sheet (Fig 1; Fierro-Monti &
Mathews, 2000; St Johnston et al, 1992).These domains occur mostly
in multiple copies (up to five) and have so far been found in 388
eukaryotic proteins, 72 of which are human (data taken from the
SMART database; Letunic et al, 2004).These proteins have an essential
role in RNA interference, RNA processing, RNA localization,
RNA editing and translational repression (Doyle & Jantsch, 2002;
Saunders & Barber, 2003).
So far, only three structures of dsRBMs in complex with dsRNA
have been determined (Table 1): a 1.9 Å crystal structure of the second
dsRBM of Xenopus laevis RNA-binding protein A (Xlrbpa2)
bound to two coaxially stacked dsRNA molecules, each 10 bp long
(Ryter & Schultz, 1998); a nuclear magnetic resonance (NMR) structure
of the third dsRBM from the Drosophila Staufen protein in complex
with a symmetrical GC-rich 12-bp duplex capped by a UUCG
tetraloop (Ramos et al, 2000); and an NMR structure of the dsRBM of
Rnt1p (an RNase III homologue from budding yeast) bound to a
14-bp RNA duplex capped by an AGAA tetraloop (Wu et al, 2004).
All three structures have several common features that reveal how a
dsRBM is able to bind to any dsRNA but not to dsDNA, regardless of
its base composition. The dsRBMs interact along one face of the
RNA duplex through both α-helices and their β1–β2 loop (Fig 1).The
contacts with the RNA cover 15 bp that span two consecutive minor
grooves separated by a major groove. In all three structures, the contacts
to the sugar-phosphate backbone of the major groove and of
one minor groove (Fig 1) are mediated by the β1–β2 loop and the
amino-terminal part of α-helix 2. These interactions are nonsequence-specific
as they involve 2’-hydroxyls and phosphate oxygens
and are perfectly adapted to the shape of an RNA double helix.
By contrast, the interactions mediated by α-helix 1 are different in all
three complexes. In the dsRBM of Xlrbpa2, α-helix 1 interacts nonspecifically
with the other minor groove of the RNA (Fig 1A), with
a few contacts to the bases. In the dsRBM of Staufen, α-helix 1
33
Institute for Molecular Biology and Biophysics,Swiss Federal Institute of Technology Zürich,
ETH-Hönggerberg, CH-8093 Zürich, Switzerland
+
Corresponding author. Tel: +41 (0)1 63 33940; Fax: +41 (0)1 63 31294;
E-mail: allain@mol.biol.ethz.ch
Submitted 22 September 2004; accepted 26 November 2004
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review
EMBO reports VOL 6 | NO 1 | 2005 ©2005 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
RNA recognition by RBPs
R. Stefl et al.
34
interacts with a UUCG tetraloop that caps the RNA double helix.
Although the UUCG tetraloop is not a natural substrate of Staufen,
this finding led to the proposal that α-helix 1 modulates the specificity
of individual dsRBMs (Ramos et al, 2000). Indeed, this was
recently confirmed by the structure of the dsRBM of Rnt1p bound to
its natural RNA substrate (Fig 1B), in which α-helix 1 recognizes the
specific shape of the minor groove created by the conserved AGNN
tetraloop (Wu et al, 2004). The α-helix 1, the conformation of which
is stabilized by an additional carboxy-terminal α-helix 3 (Fig 1B;
Leulliot et al, 2004), is tightly inserted into the RNA minor groove
and contacts the sugar-phosphate backbone and the two nonconserved
tetraloop bases, whereas the conserved A and G bases are
not involved in the interactions (Wu et al, 2004). This structure illustrates
how this dsRBM recognizes the specific shape of its RNA
target but not its sequence. dsRBMs are highly conserved and have
the same structural framework, but are chemically distinct through
variations in key residues. The structure of the dsRBM of Rnt1p in
complex with RNA highlights the essential role of the α-helix 1 in
the recognition of structured elements that deviate from regular
dsRNA. The α-helix 1 is the least-conserved secondary structure element
among various dsRBMs and seems to have a different spatial
arrangement relative to the rest of the domain in different dsRBMs.
This variability may be an important factor as many biochemical
experiments have shown that dsRBM-containing proteins have binding
specificity for a variety of RNA structures, such as stem–loops,
internal loops, bulges or helices with mismatches (Doyle & Jantsch,
2002; Fierro-Monti & Mathews, 2000; Ohman et al, 2000; Stephens
et al, 2004). Clearly, further structures are needed to decipher the
extent of RNA shape-dependent recognition by dsRBMs.
RNA sequence- and shape-dependent recognition by an RRM
The RRM is the most common RNA-binding motif. It is a small protein
domain of 75–85 amino acids with a typical βαββαβ topology
that forms a four-stranded β-sheet packed against two α-helices
(Mattaj, 1993). RRMs are found in about 0.5%–1% of human genes
(Venter et al, 2001) and are often present in multiple copies (up to six
per protein). RRM-domain-containing proteins are involved in many
cellular functions, particularly messenger RNA and ribosomal RNA
processing, splicing and translation regulation, RNA export and
RNA stability (Dreyfuss et al, 2002).
So far, ten structures of an RRM in complex with RNA have been
determined using either NMR spectroscopy or X-ray crystallography
(Table 1). These structures reveal the complexity of protein–RNA
recognition mediated by the RRM, which often involves not only
protein–RNA interactions but also RNA–RNA and protein–protein
interactions. All ten structures reveal some common features. The
main protein surface of the RRM involved in the interaction with the
RNA is the four-stranded β-sheet, which usually contacts two or three
nucleotides (exemplified here by the RRM1 of sex-lethal; Fig 2A;
Handa et al, 1999). The nucleotides are located on the surface of the
β-sheet, with the bases oriented parallel to the β-sheet plane and
often packed against conserved hydrophobic side-chains (usually
aromatics). These two or three nucleotides are recognized sequencespecifically
by interactions with the protein side-chains of the β-sheet
and with the main-chain and side-chains of the residues carboxyterminal
to the β-sheet. Interestingly, it seems that almost all possible
sequences (doublets or triplets) can be accommodated on such a
surface as the RNA sequences are different in each structure (Table 1).
Often, RRM-containing proteins bind more than three
nucleotides and recognize longer single-stranded RNA (for example,
poly(A)-binding protein (PAPB; Deo et al, 1999), sex-lethal
(Handa et al, 1999), Hu protein D (HuD; Wang & Hall, 2001),
heterogeneous nuclear RNP A1 (hnRNP A1; Ding et al, 1999),
nucleolin (Allain et al, 2000; Johansson et al, 2004), RNA
stem–loops U1A (Oubridge et al, 1994), U2B’’ (Price et al, 1998),
nucleolin (Allain et al, 2000)) or even internal loops (U1A; Allain
et al, 1997; Varani et al, 2000), all with high affinity (Kd
≈ 10–9
M–1
).
In U1A, U2B’’, nucleolin and sex-lethal, two loops between the
secondary-structure elements of the RRM (the β2–β3 loop and
the β1–α1 loop) are essential for additional contacts with the RNA
(Fig 2B). These loops vary significantly in size and amino-acid
sequence between the different RRMs. In the RRM of CBP20, the
C- and N-terminal extensions (which are stabilized by the cognate
α-helix 1
α-helix 1
β-helix 3
α-helix 2
α-helix 2
β1–β2 loop
β1–β2 loop
A
B
Fig 1 | Double-stranded RNA recognition by double-stranded RNA-binding
motifs. (A) The double-stranded RNA-binding motif (dsRBM) of Xlrbpa2
bound to dsRNA (Ryter & Schultz, 1998). The α-helix 1 (in red), aminoterminal
part of α-helix 2, and β1–β2 loop recognize non-sequencespecifically
the shape of dsRNA. Backbones are coloured blue and light blue for
two co-axially stacked duplexes. (B) The dsRBM of Rnt1p bound to the
stem–loop closed by the AGNN tetraloop (backbone in yellow and bases in
black; Wu et al, 2004). The α-helix 1, a key element for shape-specific
recognition by dsRBMs, is highlighted in red. The dsRBM of Rnt1p has an
additional carboxy-terminal α-helix 3 (in black), that modulates the
conformation of α-helix 1 (Leulliot et al, 2004). Side chains involved in
intermolecular interactions are shown.
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RNA recognition by RBPs
R. Stefl et al.
35
protein CBP80) provide a tight binding pocket for the 5’ capped
RNAs (7-methyl-G(5’)ppp(5’)N, where N is any nucleotide; Fig 2C;
Mazza et al, 2002). In proteins that contain several RRMs, highaffinity
binding can only be achieved by the cooperative binding
of at least two RRMs to the RNA (for example, in nucleolin
(Fig 2D), PABP and sex-lethal). In addition to the β-sheet–RNA
contacts, interactions between the inter-domain linker and the
RNA and between the RRMs themselves contribute to the marked
increase in affinity compared with the binding of the individual
domain alone. These structures show that the RRM is a platform
with a large capacity for variation in order to achieve high RNAbinding
affinity and specificity. For example, it is remarkable that a
single domain like nucleolin RRM2 contacts only two nucleotides,
whereas U1A RRM1 contacts 12 nucleotides and the RRM of Y14
(Fribourg et al, 2003) does not contact RNA but rather another protein.
This fascinating plasticity of the RRM explains why it is so
abundant and why it is involved in so many different biological
functions; however, this plasticity makes it difficult to predict how
the RRM achieves RNA recognition.
RNA recognition by zinc fingers
CCHH-type zinc-finger domains are the most common DNA-binding
domain found in eukaryotic genomes. Typically, several fingers are
used in a modular fashion to achieve high sequence-specific recognition
of DNA (Miller et al, 1985). Each finger displays a ββα protein fold
in which a β-hairpin and an α-helix are pinned together by a Zn2+
ion.
DNA-sequence-specific recognition is achieved by the interactions
between protein side-chains of the α-helix (at position –1, 2, 3 and 6,
for the canonical arrangement) and the DNA bases in the major
groove (Fig 3A; Wolfe et al, 2000). However, there is increasing evidence
that zinc fingers are also used to recognize RNA (Finerty & Bass,
1997; Mendez-Vidal et al, 2002; Picard & Wegnez, 1979;Theunissen
et al, 1992). The crystal structure of three zinc fingers (fingers 4–6) of
transcription factor IIIA (TFIIIA) in complex with a 61-nucleotide fragment
of the 5S RNA (Lu et al, 2003) provided the first insight into RNA
recognition by CCHH-type zinc fingers. In this structure, finger 4 binds
to loop E, finger 5 to helix V, and finger 6 to loop A (Fig 3B). Finger 4
recognizes loop E by specifically interacting with a bulged guanosine
(Fig 3C) and, similarly, finger 6 recognizes loop A by specifically
Table 1 | Various structures of RNA-binding proteins bound to RNA
Complex RNA secondary No.of RBDs in RNA sequence Function Reference; Protein Data Bank
structure structures vs in recognized specifically (PDB) ID
full-length protein by RRM β-sheet
dsRBM type
Second dsRBM Duplexa
1/3 – hnRNP association, Ryter & Schultz, 1998; 1DI2
of Xlrbpa2 translation repression
Third dsRBM Stem–loopb
1/5 – mRNA localization, Ramos et al, 2000; 1EKZ
of Staufen translation control
dsRBM of Rnt1p Stem–loop 1/1 – RNA processing Wu et al, 2004; 1T4L
RRM type
N-terminal RRM Stem–loop 1/2 CAG Pre-mRNA splicing Price et al, 1998; 1A9N
of U2B’’(in U2B’’–
U2A’–RNA complex)
N-terminal RRM Stem–loop 1/2 CAC Pre-mRNA splicing Oubridge et al, 1994; 1URN
of U1A
N-terminal RRM Internal loop 1/2 CAC Pre-mRNA splicing Allain et al, 1997; 1AUD,
of U1A Varani et al, 2000; 1DZ5
Two N-terminal Stem–loop 2/4 RRM1-CG Ribosome biogenesis Allain et al, 2000; 1FJE,
RRMs of nucleolin RRM2-UC Johansson et al, 2004; 1RKJ
Two N-terminal Single strand 2/4 RRM1-AAA Translation initiation Deo et al, 1999; 1CVJ
RRM of PABP RRM2-AAA
Two RRMs Single strand 2/2 RRM1-UUU Alternative splicing Handa et al, 1999; 1H2T
of sex-lethal RRM2-UGU
Two N-terminal Single strand 2/3 RRM1-UUU mRNA stability, Wang & Hall, 2001;
RRMs of HuD RRM2-UU translation regulation 1FXL, 1G2E
RRM of CBP20 Single strand 1/1 m7GpppG Maturation of pre-mRNA Mazza et al, 2002; 1H2V
and U-rich snRNA
Zinc finger
Fourth, fifth and sixth Truncated 3/9 – Transcription regulation Lu et al, 2003; 1UN6
zinc fingers (CCHH- 5S RNAc
type) of TFIIIA
First and second Single strand 2/2 – RNA processing Hudson et al, 2004; 1RGO
zinc fingers (CCCH- and degradation
type) of TIS11d
a
Two coaxially stacked dsRNA (each 10-bp long). b
12-bp duplex capped by a non-physiologically relevant UUCG tetraloop. c
Consists of loop A, loop E and helices I, IV andV.
CBP20, cap-binding protein 20; dsRBM, double-stranded RNA-binding motif; hnRNP, heterogeneous nuclear ribonucleoprotein; HuD, Hu protein D; PABP, poly(A)-binding protein;
RBD, RNA-binding domain; Rnt1p, RNase III homologue; RRM, RNA-recognition motif; TFIIIA, transcription factor IIIA; Xlrbpa2, Xenopus laevis RNA-binding protein A.
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RNA recognition by RBPs
R. Stefl et al.
36
interacting with two bases (an adenine and a cytosine) that also bulge
out from the rest of the RNA (Fig 3B). The specific recognition of the
RNA by both fingers 4 and 6 is achieved by side-chain contacts from
the N-terminal parts of the α-helix (at position –1, 1 and 2; Fig 3C).The
interaction of finger 5 with helixV differs from the ones made by fingers
4 and 6. In this case, finger 5 recognizes a short RNA double helix by
multiple contacts between basic amino acids of the α-helix and the
RNA sugar-phosphate backbone (Fig 3D).
In contrast to the above-mentioned CCHH zinc fingers, another
class of zinc fingers (CCCH-type) was recently found to adopt a different
fold and to recognize sequence-specifically single-stranded
RNA (Hudson et al, 2004). In this NMR structure, sequence-specific
A
B
C
D
N-terminal
N-terminal
N-terminal
m7GpppG
N-terminal
β2–β3 loop
β1–α1 loop
C-terminal
C-terminal
C-terminal
Interdomain
linker
C-terminal
3'
5'
Fig 2 | RNA recognition by RNA-recognition motifs. The similarities and differences are highlighted in red and yellow, respectively. (A) RNA-recognition
motif 1 (RRM1) of sex-lethal (shown as a ribbon model) interacts with the triplet UUU (shown as a stick model; Handa et al, 1999). The four-stranded
β-sheet (in red) recognizes three nucleotides—this is the canonical mode of RRM–RNA interaction. (B) RRM1 of U1A bound to an RNA internal loop
(Allain et al, 1997). The four-stranded β-sheet (in red) recognizes three nucleotides. The β2–β3 loop and the β1–α1 loop (in yellow) contact additional RNA
residues. (C) RRM of CBP20 complexed with m7
GpppG (7-methyl-G(5’)ppp(5’)G; Mazza et al, 2002). C- and N-terminal extensions (in yellow) provide
additional protein–RNA contacts that creates a specific binding pocket for 5’ capped RNAs. (D) RRMs 1 and 2 of nucleolin bound to an RNA stem–loop
(Allain et al, 2000). The four-stranded β-sheet of RRM2 (in red) recognizes only two nucleotides. The interdomain linker (in yellow) participates in the
recognition of the hairpin architecture.
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RNA recognition by RBPs
R. Stefl et al.
37
RNA recognition is achieved by a network of intermolecular hydrogen
bonds between the protein main-chain functional groups and
the Watson–Crick edges of the bases (Hudson et al, 2004). These
structures reveal that zinc fingers bind to RNA differently to the way
they do to DNA. The CCHH-type zinc fingers have two modes of
RNA binding. First, the zinc fingers interact non-specifically with the
backbone of a double helix, and second, the zinc fingers specifically
recognize individual bases that bulge out of a structurally rigid element.The
CCCH-type zinc fingers show a third mode of RNA binding,
in which the single-stranded RNA is recognized in a sequence-specific
manner. Taken together, zinc fingers represent a unique class of
nucleic-acid-binding proteins that are capable of a direct readout
of the DNA sequence within a DNA double helix, a direct readout of
the RNA sequence within single-stranded RNA, and an indirect
readout of the RNA as they recognize the shape of the RNA rather
than its sequence. Of course, more structures of CCHH-type and
CCCH-type zinc fingers in complex with RNA will need to be
determined to generalize their mode of RNA recognition.
Conclusions
Proteins that contain RNA-binding domains and their interactions
with RNA have important roles in all aspects of gene expression and
regulation. The enormous diversity of interactions observed in protein–RNA
complexes indicates that a simple recognition code is
unlikely to exist in the world of protein–RNA interactions. However,
two unifying themes may be inferred from the known complexes: the
recognition of the primary RNA sequence and/or the recognition of
the RNA shape by individual RBPs. In a simplistic view, the RRMs,
dsRBMs and CCHH-type zinc fingers seem to be shaped to recognize
single-stranded RNA, double-stranded RNA and RNA bulges, respectively.
However, we have shown here by reviewing several recent
protein–RNA complex structures that, the RRM and, to a lesser
extent, the dsRBMs and the CCHH-type zinc fingers have evolved to
recognize specifically a rich repertoire of RNAs in terms of length,
sequence and structure. This is achieved in three ways: first, by the
subtle amino-acid change in variable regions of the domains, namely
the β2–β3 and the β1–α1 loops in the RRM, α-helix 1 in the dsRBM
and the α-helix in the zinc fingers; second, by multiplication of the
domains to achieve higher affinity through cooperative binding; and
third, by extension of the protein domain. Although more structures
still need to be determined, it might soon be possible to predict which
RBP binds to which RNA, and how it recognizes its target. As a consequence,
post-transcriptional gene expression and its regulation could
be understood and controlled at the atomic level.
F4
F6
F5
F4
F5
LoopE
Loop E
HelixV
Helix V
Loop A
A B
C
D
Fig 3 | DNA vs RNA recognition by CCHH-type zinc fingers. (A) Zinc finger 2 of Zif268 bound to double-stranded DNA (Pavletich & Pabo, 1991). The α-helix of
the zinc finger (in red) inserts into the DNA major groove; base contacts are made from positions –1, 2, 3 and 6 of the α-helix (the protein side-chains are shown as
an element-type coloured stick model). The DNA bases that are recognized by the finger are coloured yellow. (B) Overall view of the complex of transcription
factor IIIA (TFIIIA) fingers 4–6 (F4–F6) and 61-nucleotide 5S RNA (Lu et al, 2003). The protein and RNA are represented as ribbon models. The bulged bases
involved in the recognitions are highlighted in yellow. Cyan balls represent zinc ions. (C) TFIIIA finger 4 (F4) bound to loop E (Lu et al, 2003). The α-helix (in red)
of the finger 4 specifically interacts with a guanosine base that bulges out (in yellow); the base contacts are made from the side-chain at position –1, 1 and 2 of the
α-helix. (D) TFIIIA finger 5 (F5) bound to helixV (Lu et al, 2003). The α-helix (in red) of finger 5 recognizes the dsRNA shape by non-sequence-specific contacts
to the RNA sugar-phosphate backbone.
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RNA recognition by RBPs
R. Stefl et al.
38
ACKNOWLEDGEMENTS
We apologize to authors whose work could not be cited due to space
constraints. The authors are supported by the Swiss National Science
Foundation (No. 31-67098.01), the Roche Research Fund for Biology at the
ETH Zurich (F.H.-T.A.), and the European Molecular Biology Organization
and the Human Frontier Science Program postdoctoral fellowships (R.S.).
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Richard Stefl,Lenka Skrisovska & Frédéric H.-T.Allain,
who is an EMBO Young Investigator
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