Fas-activated serine/threonine kinase is an enzyme that in humans is encoded by the FASTK gene.
The protein encoded by this gene is a member of the serine/threonine protein kinase family. This kinase was shown to be activated rapidly during Fas-mediated apoptosis in Jurkat cells. In response to Fas receptor ligation, it phosphorylates the apoptosis-promoting nuclear RNA-binding protein TIA1. The encoded protein is a strong inducer of lymphocyte apoptosis.
Two transcript variants encoding different isoforms have been found for this gene. The longer one is located in the cell nucleus and cytoplasm, and the shorter one is 34 amino acids at the N-terminus, which is transported to the mitochondrion.
The mitochondrial FASTK isoform localizes in the mitochondrial granules, where it interacts with the GRSF1 protein and binds to mt-mRNA ND6, the template of the only protein encoded on the light strand of the mitochondrial genome. In the case of FASTK deletion, it was observed loss of ND6 mt-mRNA and a 50-60% decrease in the activity of complex I of the respiratory chain. However, overexpression of this protein resulted in the stabilization of ND6 mt-mRNA
Other variants exist, but their full-length natures have not yet been determined.
