Protein Structure and Function Lecture 6 Notes: Post-Translational Modifications Post-Translational Modifications of Proteins - Post-translational modifications (PTMs) are covalent modifications of protein structure (e.g. elaborations with new structures) post-translation. - SARSCov2 is a spike protein (trimer) at the infection head of the virus which helps to infect host cells. After post-translational modification, many new structures are added to the spike protein. For example, fatty acids are added to the tail of the protein to make it compatible with the membrane to which it is anchored. - The main post-translational modifications, the amino acids (side chains) that they affect, and the functional group include: (a) Deamidation to Asp or iso(Asp)/N-linked glycosylation (CONH2 on Asn) (b) Phosphorylation (NH on His) (c) O-linked glycosylation (OH on Ser/Thr) (d) Glycosylation (OH on Tyr) (e) N-acetylation (NH2 on Ala, at N-terminus) (f) Ubiquitination (NH2SH on Lys) - Proteolysis, the processing of a protein via cleavage, is also a PTM because it is a covalent modification to the protein. An example of this is the action of signal sequences. - PTM increases protein diversity. From around 20-25,000 genes, pre-transcription modification such as alternative splicing, alternative promoters and mRNA editing increase proteome complexity to around 100,000 transcripts. PTMs create over 1 million proteins for many different functions. - PTMs can be used as a means of pathogenesis. For example, many bacterial pathogens secrete virulence factors (effector proteins) directly into host cells. These effectors suppress pro-inflammatory host signalling while promoting bacterial infection. - A subset of these effector proteins can post-translationally modify host proteins using novel chemistry that is not otherwise found in the mammalian proteome. This includes: phosphate-Ăź elimination, phosphoribosyl-linked ubiquitination, serine/threonine acetylation, glutamine deamidation and many others. - The distribution of PTMs across amino acids reveals that serine is the most post- translationally modified amino acid. Following this, threonine, tyrosine and lysine are the next most popular. - Post-translational proteolysis occurs to introduce diversity (e.g. removal of N-terminal Met directly after protein synthesis). It is also involved in the translocation of proteins across membranes (mediated by signal sequences) for further processing, control of protein function by switching between active and inactive forms (e.g. insulin, chymotrypsin), as well as mediating signalling pathways. - Removal of N-terminal Met is carried out co-translationally by the enzyme methionine aminopeptidase (MAPs). They have a specificity which mainly depends on the size of the side chain and adjacent amino acid (e.g. if bigger than 1.2flĂ…, there is no cleavage of the 3rd aa). - The removal of signal sequences enables translocation to the ER for further processing (e.g. glycosylation). Translocation can be co-translational or post-translational. The signal peptide is cleaved once inside the ER, and the truncated protein undergoes PTMs.
- Signal sequences (around 16-30 amino acids long) are usually a stretch of hydrophobic amino acids (compatible with the hydrophobic ER membrane) present at the N-terminus of the majority of newly synthesised proteins which are destined towards secretion. - Insulin is biosynthesised as proinsulin, translocated to the ER (mediated by signal peptides)