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Introduction

Geobacillus stearothermophilus's L14 ribosomal protein structure.

The L14 ribosomal protein is a protein which is included in the 50S subunit of the procaryote ribosome and in the 60s subunit of the eucaryote ribosome.The L14 ribosomal protein take part in an operon in Procaryotic and on the third human's chromosome. The 50S and 60S subunits are the twos biggest subunits of the prokaryotic and eukaryotic ribosomes. It is a cytoplasmic protein which contains a basic region leucine zipper which participates to the mechanism of translation by allowing the folding and stabilization of rRNA. The L14 subunit is one of the most conserved protein in the ribosome in a lot of species. For example, 66% sequence identity exists between the Escherichia coli and Bacillus stearothermophilus molecules. L14 has been located on the 50S and 30S subunit interface between peptidyl transferase and GTPase regions. It allows contacts with the 16S rRNA of the 30S subunit (bridges B5 and B8) connecting the 2 subunits.

Structure

The L14 subunit has a molecular mass of 13.3 kDa and contains 122 amino acids. The molecule is extremly compact so water molecule are exclude. It belong to α+β class of proteins. It is composed of five stranded  : β1-β2-β3-β4-β7 all antiparallel, stabilized by hydrogen-bounding, a C-terminal region which contains the two small and a beta-ribbon : the extended loop8. The beta-barrel contains a hydrophobic core of highly conserved residues : Ala, Leu, Ile, Val, Cys. The beta-barrel is stabilized by some hydrogen-bonding such as a bonding between loop 2 and loops 4 and 8. . The loops 3 which connects β2 and β3 allows to close this end with the hydrophobic side chains. The top of the β-barrel is doing by 2 valines 51 and 54 in loop 4 The loop 2 have an unusual and highly structure turn which contains the most conserved sequence, stabilized by a complexe hydrogen-bonding : the alide protons and carbonyl oxygène of 3 residues and a water molecule. The turn allow to stabilize loop 4 and loop 8. Loop8 has several interactions. Finally, three structural waters (206–208) are involved in stabilizing the local conformation around residue 91 in loop8 and the N terminus of helix α2.

Bidding sites

Protein-protein interactions are essential for the stability of ribosomes. The L14 subunits presents a perfect hydrophobic area on its structure too allow such an interaction with other ribosome’s subunits. This area is on the beta-barrel and is composed of  : Leu25, Val40, Val57 and Ile2. This area is very exposed and separated from the RNA binding site. Links to L4, L7/L12, L10, L11, L17 and L19 has been showed and are allowed by this hydrophobic structure.

In the L14 subunit, there are also two binding sites for the fixation of the rRNA. These two sites could each bind to a specific RNA sequence and induce the folding of the 23S rRNA.

Role of the L14 subunit

The ribosome orchestrates the synthesis of proteins in all cells.The rRNA three dimensional organization is a major element in the activity of the ribonucleoprotein complex. This three dimensional structure is organized by the ribosomal proteins. Sequence alignment show that the structure of the L14 I highly conserved. It’s probably due to the fact that both mechanism and structure of the ribosome are common in all organisms.

References

Christopher Davies, Stephen W White, V Ramakrishnan. The crystal structure of ribosomal protein L14 reveals an important organizational component of the translational apparatus.

By Bonhomme Clémence and Dilda Nathan