The Plastid Ribosomal Proteins

Abstract

We have completed identification of all the ribosomal proteins (RPs) in spinach plastid (chloroplast) ribosomal 50 S subunit via a proteomic approach using twodimensional electrophoresis, electroblotting/protein sequencing, high performance liquid chromatography purification, polymerase chain reaction-based screening of cDNA library/nucleotide sequencing, and mass spectrometry (reversed-phase HPLC coupled to electrospray ionization mass spectrometry and electrospray ionization mass spectrometry). Spinach plastid 50 S subunit comprises 33 proteins, of which 31 are orthologues of Escherichia coli RPs and two are plastid-specific RPs The plastid (chloroplast) ribosome is a plant-specific, organelle ribosome that produces proteins encoded by the plastid genome. Plastid ribosomes are responsible for the synthesis of huge amounts of biomass, since the large subunit of ribulose 1,5-bisphophate carboxylase/oxygenase (a most abundant protein in the biosphere) is synthesized in plastids. Plastid ribosomes are very similar to the eubacterial 70 S-type ribosome, in composition and general mode of function (1-4). The rRNAs and most of the characterized ribosomal proteins (RPs) 1 in plastid ribosomes also bear close resemblance to the corresponding components so far identified in cyanobacteria, a correlation supporting the importance of endosymbiotic theory in plastid evolution (5).The Escherichia coli ribosome, the most well studied of the eubacterial ribosomes (6), is composed of 21 RPs in the 30 S subunit and 33 RPs in the 50 S subunit. Two more possible E. coli RPs have been suggested: protein Y, the product of E. coli yfia gene (7), bearing a distant sequence homology to a chloroplast-specific RP (PSRP-1), and a protein designated S22 (8). Post-translational modifications are found in many E. coli RPs, although a modification in L16 (Arg 81 ) remains yet to be characterized (see "Results" for plastid L16). We have recently identified all the RPs in spinach plastid 30 S ribosomal subunit, including all its PSRPs and many post-translational modifications (83). The number of RPs in plastid 50 S subunits has so far only been estimated (ϳ35-39; reviewed in Ref.2) and has not been determined.Although the constituents of plastid translational machinery in general are similar to those of E. coli, the genes are distributed in two genome compartments: the plastid and the nucleus. The rRNA and tRNA genes are located in the plastid genome, whereas the genes for processing/modification enzymes, aminoacyl tRNA synthetases, and 60% of the RPs are located in the nuclear genome (1-4). The plastid translation system also differs from the eubacterial system in other significant ways, e.g. chloroplast mRNA is often edited (9); about 60% of chloroplast mRNAs lack canonical ribosome-binding sites found in E. coli mRNAs (10); mRNA levels in chloroplasts remain relatively unchanged through dark/light transitions, whereas protein

The Plastid Ribosomal Proteins

“…They (six in all: four in the 30 S subunit, and two in the 50 S subunit; Ref. 54) are designated PSRP (plastid-specific ribosomal protein), PSRP-1-PSRP-4 in the 30 S subunit; the numbering based on pI and molecular mass. The PSRP nomenclature is recommend over the previous designations, e.g.…”

“…A summary of the results from this study are presented below, including identification of all the proteins in the plastid 30 S subunit, post-translational modifications, and identification of two additional novel PSRPs. A corresponding study of the plastid 50 S subunit is presented in the accompanying paper (54).…”

A Proteomics Approach to the Identification of Mammalian Mitochondrial Small Subunit Ribosomal Proteins

“…Since there are 21 proteins in the bacterial ribosome, we have begun designating the new mammalian mitochondrial ribosomal proteins beginning at MRP-S22. This approach to the nomenclature of organellar ribosomal proteins has recently been adopted for chlo- roplast ribosomal proteins (22,23).…”

Cryo-EM study of the spinach chloroplast ribosome reveals the structural and functional roles of plastid-specific ribosomal proteins

“…In addition, the chloro-ribosome possesses six nonorthologous proteins, which are termed ''plastid-specific ribosomal proteins'' (PSRPs): four of these proteins (PSRP1-4) are associated with the 30S subunit, and two (PSRP5 and PSRP6) are associated with the 50S subunit. The larger size of the PRPs as well as the presence of six PSRPs leads to a significantly altered protein/RNA mass ratio of 2/3 in the case of chlororibosomes, as compared with 1/3 for E. coli (5,6). The higher protein content of chloro-ribosomes, and especially the presence of PSRPs, has prompted the suggestion that chloroplasts have evolved specific proteins that play unique functional roles during chloroplast translation, such as the light-dependent stimulation of protein synthesis (5-7).…”

“…Proteomic studies reveal that the chloro-ribosomes possess in total 58 ribosomal proteins, 25 and 33 proteins in the 30S and 50S subunits, respectively (5,6), and have orthologs to all 52 E. coli ribosomal proteins, with the exception of L25 and L30. Plastid ribosomal proteins (PRPs) generally are larger than their E. coli counterparts, containing extensions predominantly at the N and C termini.…”