Techniques for studying protein heterogeneity and post-translational modifications

Proteins often undergo several post-translational modification steps in parallel to protein folding. These modifications can be transient or of a more permanent nature. Most modifications are, however, susceptible to alteration during the lifespan of proteins. Post-translational modifications thus generate variability in proteins that are far beyond that provided by the genetic code. Co- and post-translational modifications can convert the 20 specific codon-encoded amino acids into more than 100 variant amino acids with new properties. These, and a number of other modifications, can considerably increase the information content and functional repertoire of proteins, thus making their analysis of paramount importance for diagnostic and basic research purposes. Various methods used in proteomics, such as 2D gel electrophoresis, 2D liquid chromatography, mass spectrometry, affinity-based analytical methods, interaction analyses, ligand blotting techniques, protein crystallography and structure-function predictions, are all applicable for the analysis of these numerous secondary modifications. In this review, examples of some of these techniques in studying the heterogeneity of proteins are highlighted. In the future, these methods will become increasingly useful in biomarker searches and in clinical diagnostics.