STK4encodesacytoplasmickinasewithastructuresimilartoyeastSte20pkinase(anenzymethatcatalyzesthetransferofphosphategroupsfromhigh-energy,phosphate-donatingmoleculestospecificsubstrates).
Serine/threonine-protein kinase 4 is an enzyme that in humans is encoded by the STK4 gene. The protein encoded by this gene is a cytoplasmic kinase that is structurally similar to the yeast Ste20p (sterile 20 protein) kinase, which acts upstream of the stress-induced mitogen-activated protein kinase (MAPK) cascade. The encoded protein can phosphorylate myelin basic protein and undergoes autophosphorylation. In chemistry, phosphorylation (a phosphate group is added to a molecule) of a molecule is the attachment of a phosphoryl group. Together with its counterpart, dephosphorylation, (to lose a molecule) it is critical for many cellular processes in biology. Protein phosphorylation is especially important for their function; for example, this modification activates (or deactivates) almost half of the enzymes present in Saccharomyces cerevisiae, thereby regulating their function. 
Autophosphorylation is a type of post-translational modification of proteins. It is generally defined as the phosphorylation of the kinase by itself. In eukaryotes, this process occurs by the addition of a phosphate group to serine, threonine or tyrosine residues within protein kinases, normally to regulate the catalytic activity. Autophosphorylation may occur when a kinases' own active site catalyzes the phosphorylation reaction (cis autophosphorylation), or when another kinase of the same type provides the active site that carries out the chemistry (trans autophosphorylation). In general, the phosphate groups introduced are gamma phosphates from nucleoside triphosphates, most commonly ATP. 
