tRNAs serve as adaptors that align amino acids on the mRNA template, where peptide bond formation is catalyzed by rRNA. A variety of nonribosomal proteins are also required for initiation, elongation, and termination of translation. Translation is initiated by the binding of initiator tRNA and mRNA to the small ribosomal subunit. The large ribosomal subunit then joins the complex, and the polypeptide chain elongates until the ribosome reaches a termination codon. Translation of specific mRNAs can be regulated by repressor proteins and miRNAs. In addition, global translational activity can be regulated by modification of initiation factors.
Protein folding is facilitated by chaperones and at least two types of enzymes, protein disulfide isomerase and peptidyl prolyl isomerase. Aggregation of misfolded proteins leads to a variety of diseases, including Alzheimer's. The processing of many proteins involves proteolysis, glycosylation, and the addition of lipids.
Proteins are regulated by the binding of small molecules, phosphorylation and other reversible modifications, and interactions with other proteins. Proteins can also be targeted for selective degradation by the addition of ubiquitin, followed by rapid proteolysis in the proteasome.