Crystal structure of the ffh and EF-G binding sites in the conserved domain IV of Escherichia coli 4.5S RNA.
BACKGROUND
Bacterial signal recognition particle (SRP), consisting of 4.5S RNA and Ffh protein, plays an essential role in targeting signal-peptide-containing proteins to the secretory apparatus in the cell membrane. The 4.5S RNA increases the affinity of Ffh for signal peptides and is essential for the interaction between SRP and its receptor, protein FtsY. The 4.5S RNA also interacts with elongation factor G (EF-G) in the ribosome and this interaction is required for efficient translation.


RESULTS
We have determined by multiple anomalous dispersion (MAD) with Lu(3+) the 2.7 A crystal structure of a 4.5S RNA fragment containing binding sites for both Ffh and EF-G. This fragment consists of three helices connected by a symmetric and an asymmetric internal loop. In contrast to NMR-derived structures reported previously, the symmetric loop is entirely constituted by non-canonical base pairs. These pairs continuously stack and project unusual sets of hydrogen-bond donors and acceptors into the shallow minor groove. The structure can therefore be regarded as two double helical rods hinged by the asymmetric loop that protrudes from one strand.


CONCLUSIONS
Based on our crystal structure and results of chemical protection experiments reported previously, we predicted that Ffh binds to the minor groove of the symmetric loop. An identical decanucleotide sequence is found in the EF-G binding sites of both 4.5S RNA and 23S rRNA. The decanucleotide structure in the 4.5S RNA and the ribosomal protein L11-RNA complex crystals suggests how 4.5S RNA and 23S rRNA might interact with EF-G and function in translating ribosomes.
Jovine
L
L
MRC Laboratory of Molecular Biology, Cambridge, England. jovinl02@doc.mssm.edu
Hainzl
T
T
Oubridge
C
C
Scott
W G
WG
Li
J
J
Sixma
T K
TK
Wonacott
A
A
Skarzynski
T
T
Nagai
K
K
D001426:Bacterial Proteins / Q000378:metabolism*; D020029:Base Pairing; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D019281:Dimerization; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D006150:Guanine Nucleotides / Q000737:chemistry; D008187:Lutetium / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020653:Peptide Elongation Factor G / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D012329:RNA, Bacterial; D012335:RNA, Ribosomal / Q000737:chemistry* / Q000378:metabolism; D018271:Signal Recognition Particle / Q000378:metabolism*
C409968:4.5S RNA; D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C082931:Ffh protein, E coli; D006150:Guanine Nucleotides; D020653:Peptide Elongation Factor G; D012329:RNA, Bacterial; D012335:RNA, Ribosomal; D018271:Signal Recognition Particle; D008187:Lutetium; C078170:2',3'-cyclic GMP