Cryo-EM study of the spinach chloroplast ribosome reveals the structural and functional roles of plastid-specific ribosomal proteins.
Protein synthesis in the chloroplast is carried out by chloroplast ribosomes (chloro-ribosome) and regulated in a light-dependent manner. Chloroplast or plastid ribosomal proteins (PRPs) generally are larger than their bacterial counterparts, and chloro-ribosomes contain additional plastid-specific ribosomal proteins (PSRPs); however, it is unclear to what extent these proteins play structural or regulatory roles during translation. We have obtained a three-dimensional cryo-EM map of the spinach 70S chloro-ribosome, revealing the overall structural organization to be similar to bacterial ribosomes. Fitting of the conserved portions of the x-ray crystallographic structure of the bacterial 70S ribosome into our cryo-EM map of the chloro-ribosome reveals the positions of PRP extensions and the locations of the PSRPs. Surprisingly, PSRP1 binds in the decoding region of the small (30S) ribosomal subunit, in a manner that would preclude the binding of messenger and transfer RNAs to the ribosome, suggesting that PSRP1 is a translation factor rather than a ribosomal protein. PSRP2 and PSRP3 appear to structurally compensate for missing segments of the 16S rRNA within the 30S subunit, whereas PSRP4 occupies a position buried within the head of the 30S subunit. One of the two PSRPs in the large (50S) ribosomal subunit lies near the tRNA exit site. Furthermore, we find a mass of density corresponding to chloro-ribosome recycling factor; domain II of this factor appears to interact with the flexible C-terminal domain of PSRP1. Our study provides evolutionary insights into the structural and functional roles that the PSRPs play during protein synthesis in chloroplasts.
Sharma
Manjuli R
MR
Laboratory of Structural Pathology, Division of Molecular Medicine, Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY 12201-0509, USA.
Wilson
Daniel N
DN
Datta
Partha P
PP
Barat
Chandana
C
Schluenzen
Frank
F
Fucini
Paola
P
Agrawal
Rajendra K
RK
D002736:Chloroplasts / Q000737:chemistry* / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D018360:Crystallography, X-Ray; D019143:Evolution, Molecular; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D018087:Plastids / Q000737:chemistry / Q000648:ultrastructure; D011487:Protein Conformation; D012269:Ribosomal Proteins / Q000737:chemistry* / Q000378:metabolism; D054683:Ribosome Subunits, Large, Eukaryotic / Q000737:chemistry* / Q000648:ultrastructure; D054682:Ribosome Subunits, Small, Eukaryotic / Q000737:chemistry* / Q000648:ultrastructure; D018724:Spinacia oleracea / Q000378:metabolism
D010940:Plant Proteins; D012269:Ribosomal Proteins