Protein kinases catalyze the phosphotransfer reaction fundamental to most signalling and regulatory processes in the eukaryotic cell. The catalytic subunit contains a core that is common to both serine/threonine and tyrosine protein kinases. The catalytic domain contains the nucleotide-binding site and the catalytic apparatus in an inter-lobe cleft. Structurally it shares functional and structural similarities with the ATP-grasp fold, which is found in enzymes that catalyse the formation of an amide bond, and with PIPK (phosphoinositol phosphate kinase). The three-dimensional fold of the protein kinase catalytic domain is similar to domains found in several other proteins. These include the catalytic domain of actin-fragmin kinase, an atypical protein kinase that regulates the F-actin capping activity in plasmodia; the catalytic domain of phosphoinositide-3-kinase (PI3K), which phosphorylates phosphoinositides and as such is involved in a number of fundamental cellular processes such as apoptosis, proliferation, motility and adhesion; the catalytic domain of the MHCK/EF2 kinase, an atypical protein kinase that includes the TRP (transient channel potential) calcium-channel kinase involved in the modulation of calcium channels in eukaryotic cells in response to external signals; choline kinase, which catalyses the ATP-dependent phosphorylation of choline during the biosynthesis of phosphatidylcholine; and 3',5'-aminoglycoside phosphotransferase type IIIa, a bacterial enzyme that confers resistance to a range of aminoglycoside antibiotics.