A ribosomal protein is any of the proteins that, in conjunction with rRNA, make up the ribosomal subunits involved in the cellular process of translation. A large part of the knowledge about these organic molecules has come from the study of E. coli ribosomes. Most ribosomal proteins have been isolated and specific antibodies have been produced. These, together with electronic microscopy and the use of certain reactives, have allowed for the determination of the topography of the proteins in the ribosome. E.coli, other bacteria and Archaea have a 30S small subunit and a 50S large subunit, whereas humans and yeasts have a 40S small subunit and a 60S large subunit. Equivalent subunits are frequently numbered differently between bacteria, Archaea, yeasts and humans.
The ribosome of E. coli has about 22 proteins in the small subunit (labelled S1 to S22) and 34 proteins in the large subunit (L1 to L36). All of them are different with three exceptions: one protein is found in both subunits (S20 and L26), L7 and L12 are acetylated and methylated forms of the same protein, and L8 is a complex of L7/L12 and L10. In addition, L31 is known to exist in two forms, the full length at 7.9 kilodaltons (kDa) and fragmented at 7.0 kDa. This is why the number of proteins in a ribosome is of 56. Except for S1 (with a molecular weight of 61.2 kDa), the other proteins range in weight between 4.4 and 29.7 kDa.
Recent 'de novo' proteomics experiments where the authors characterized in vivo ribosome-assembly intermediates and associated assembly factors from wild-type Escherichia coli cells using a general quantitative mass spectrometry (qMS) approach have confirmed the presence of all the known small and large subunit components and have identified a total of 21 known and potentially new ribosome-assembly-factors that co-localise with various ribosomal particles.
In the small (30S) subunit of E. coli ribosomes, the proteins denoted S4, S7, S8, S15, S17, S20 bind independently to 16S rRNA. After assembly of these primary binding proteins, S5, S6, S9, S12, S13, S16, S18, and S19 bind to the growing ribosome. These proteins also potentiate the addition of S2, S3, S10, S11, S14, and S21. Protein binding to helical junctions is important for initiating the correct tertiary fold of RNA and to organize the overall structure. Nearly all the proteins contain one or more globular domains. Moreover, nearly all contain long extensions that can contact the RNA in far-reaching regions. Additional stabilization results from the proteins' basic residues, as these neutralize the charge repulsion of the RNA backbone. Protein-protein interactions also exist to hold structure together by electrostatic and hydrogen bonding interactions. Theoretical investigations pointed to correlated effects of protein-binding onto binding affinities during the assembly process 
Table of E.coli small 30S ribosomal subunit proteins
Ribosomes, which synthesize the proteome of cells, are complex ribonucleoproteins that, in eukaryotes, contain 79–80 proteins and four ribosomal RNAs(rRNAs). General or specialized chaperonessolubilize the ribosomal proteins and facilitate their import into the nucleus. Assembly of the eukaryotic ribosome appears to be driven by the ribosomal proteins in vivo when assembly is also aided by chaperones. Most ribosomal proteins assemble with rRNA co-transcriptionally, becoming associated more stably as assembly proceeds, and the active sites of both subunits are constructed last.
Table of E.coli large 50S ribosomal subunit proteins
^Arnold R, Reilly J (Apr 1999). "Observation of Escherichia coli ribosomal proteins and their posttranslational modifications by mass spectrometry". Analytical Biochemistry269 (1). doi:10.1006/abio.1998.3077. PMID10094780.
Ban N, Beckmann R, Cate J, Dinman J, Dragon F, Ellis S et al. (Feb 2014). "A new system for naming ribosomal proteins". Current Opinion in Structural Biology24: 165–9. doi:10.1016/j.sbi.2014.01.002. PMID24524803.