The ribosome is a large, macromolecular machine that orchestrates the entire process of protein synthesis. Each human ribosome comprises four rRNAs and approximately 80 proteins, called ribosomal proteins (RPs), assembling into a 4.5-MDa structure. The ribosome is formed by two subunits; the large 60 S (S stands for Svedberg unit and refers to the sedimentation coefficient) and the small 40 S subunit join and constitute the 80 S ribosome in eukaryotic cells. rRNAs are the catalytic elements, and RPs are classically considered structural “scaffolding” units. The process of translation of an mRNA is typically divided into four phases: initiation, elongation, termination, and ribosome recycling. During the initiation phase the 40 S subunit binds to the 5′ cap of the mRNA and scans the mRNA toward its 3′-end searching for the translation start codon. The start codon is a trinucleotide sequence, usually an AUG, located within a Kozak consensus sequence (A/GNNAUGG), which is the optimal nucleotide context for translation initiation. The initiator tRNA specific for methionine, called the initiator tRNA_i^Met, binds the 40 S subunit and helps to recruit the larger 60 S subunit once the AUG is found. Next, the mRNA sequence is read in triplets by additional aminoacyl-tRNAs which mediate the elongation of the polypeptide chain (elongation phase). Translation is terminated as soon as the ribosomes reach a stop codon (termination phase). The polypeptide is then released, and ribosomal subunits detach from the mRNAs and are used for a new cycle of translation (ribosome recycling phase). Typically, multiple ribosomes are simultaneously engaged in translating a single mRNA molecule forming a polyribosome, or polysome. The phases of protein synthesis are regulated by eukaryotic initiation, elongation, and termination (or release) factors, termed eIFs, eEFs, and eRFs, respectively. eIFs are members of the guanine nucleotide-binding proteins (G protein) superfamily that function as molecular switches and promote unidirectionality of cellular processes. eIF2a also functions as a key mediator of the integrated stress response (ISR) limiting translation initiation and protein synthesis in response to stress conditions, such as amino acid or glucose starvation and viral infection.

As an example, in reticulocytes, heme starvation inhibits the syn thesis of α- and β-globin chains via regulation of eIF2. Activated protein kinase hemin-regulated inhibitor (HRI) phosphorylates the α subunit of eIF2; phosphorylated eIF2 sequesters the guanine-nucleotide exchange factor (GEF) eIF2B, limiting its availability for the exchange reaction of eIF2-GDP to eIF2-GTP, halting globin translation. Heme availability thus becomes the rate-limiting step and pre vents toxic accumulation of globin chains in the absence of heme.

A second major control point of general protein synthesis is mediated by the eIF4F heterotrimeric protein complex that makes the mRNA accessible to binding by the 40 S ribosome subunit: its cap binding protein eIF4E binds the mRNA cap, the ATP-dependent RNA helicase eIF4A unwinds structural elements in the 5′-end of mRNA, and eIF4G serves as scaffold protein in the complex. As the lowest abundance subunit, eIF4E represents the rate-limiting step in protein synthesis. In addition, inhibition of the cap-binding activity of eIF4E by eIF4E-binding proteins (eIF4EBPs) prevents assembly of the eIF4F complex.

Additional modes of regulation take advantage of the mRNA sequence itself. In the control of iron metabolism, IREs, hairpin structures formed in the UTRs of mRNAs, are bound by IRE-BPs. In iron-starved cells, IREs in the 3′UTR and 5′UTR of TfR and ferritin mRNAs, respectively, are bound by IRE-BPs that stabilize the TfR transcript while at the same time inhibiting translation initiation of ferritin (see Fig. 1). Conversely, when iron is abundant, IRE-BPs have a lower affinity to IREs, and as a result TfR mRNA is degraded whereas ferritin mRNA translation is stimulated. In this manner, cells can coordinately regulate iron uptake and iron sequestration in response to changes in iron availability.

Fig1. CONTROL OF TRANSFERRIN RECEPTOR EXPRESSION BY RNA STABILITY. The transferrin receptor messenger RNA (mRNA) has five RNA iron-responsive elements (IRE) in the 3′ untranslated region (3′UTR). Upper panel: when the iron concentration is low, the IRE-binding proteins (IRE-BPs) bind to the IRE elements and stabilize the mRNA, reading to increased protein expression. When the iron concentration is high (Fe), IRE-BPs are inactive and the transferrin receptor transcript is susceptible to degradation by endonucleases, leading to decreased protein expression. CDS, Coding sequence.

مواضيع ذات صلة

Targeting of Nuclear Proteins

Ribosome Disorders

RNA Granules and Membraneless Organelles

RNA Interference

Balancing Synthesis of Ribosomal Components

Regulation of Ribosome Synthesis

Proportionality of Ribosome Levels and Growth Rates

The Signal Recognition Particle and Translocation

Verifying the Signal Peptide Model

Directing Proteins to Specific Cellular Sites

Protein Elongation Rates

Messenger Instability