Phosphorylates the splicing regulator TIA1, thereby promoting the inclusion of FAS exon 6, which leads to an mRNA encoding a pro-apoptotic form of the receptor. Required for the biogenesis of some mitochondrial-encoded mRNAs, specifically stabilizes ND6 (NADH dehydrogenase complex subunit 6) mRNA, and regulates its levels. ATP + L-seryl-[Fas-activated protein] = ADP + H(+) + O-phospho-L-seryl-[Fas-activated protein] ATP + L-threonyl-[Fas-activated protein] = ADP + H(+) + O-phospho-L-threonyl-[Fas-activated protein] ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with TIA1; the interactions leads to TIA1 phosphorylation (PubMed:7544399, PubMed:17135269). Interacts with TIAR (PubMed:7544399, PubMed:17135269). Colocalizes with mitochondrial RNA granules. Expressed in heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. The RAP domain is essential for RNA-binding. Autophosphorylated on serine/threonine residues. Activated by dephosphorylation. Belongs to the FAST protein kinase family. Extended N-terminus.