ABSTRACT
Eukaryotic mRNA translation at ribosomes occurs in initiation, elongation, and termination phases. Exogenous input signals are controlling protein biosynthesis on both the level of gene transcription and the level of pre-messenger RNA translation. Gene expression culminates in the translation of a nucleotide sequence into the amino acid sequence of the complementary polypeptide. This translation occurs at the ribosomes and is controlled by various regulatory proteins or translation factors, which are targets of cellular signaling cascades. The ultimate goal of the unfolded-protein response is to increase the protein folding capacity of the endoplasmic reticulum by promoting the synthesis of chaperones. Exogenous signals that induce eIF4E phosphorylation also inactivate 4E-binding protein through phosphorylation, which in this case is catalyzed by the protein kinase mammalian target of rapamycin. To save energy, protein biosynthesis is suppressed in stress situations. The degradation of the extremely protease-sensitive transcription factors of the unfolded-protein response also aims at signal extinction.
