The totality of all proteins in an organism, a tissue, a cell or a cell compartment under exactly defined conditions and at a specific point in time is termed proteome. As a result of continuous de novo synthesis and the concurrent degradation of proteins, its composition changes continuously. These changes are influenced by environmental stimuli and controlled by complex regulatory processes. Depending on the species, the proteome can comprise up to 1,000,000 protein species and thus normally exceeds the number of gene sequences encoded in the genome many times over. This is primarily the result of mRNA splicing and subsequent (posttranslational) modifications of the proteins.
Posttranslational protein modifications occur subsequent to translation. These changes are usually initiated by the organism or by the cells themselves and can be influenced by environmental factors. Examples of the multifarious possibilities are phosphorylation, which is mediated by protein kinases, hydroxylations of proline residues or glycosylations. Some of these processes occur at the protein synthesis site, others occur in certain organelles or also outside the cell.