Required for normal mitochondrial respiration (PubMed:20869947). Increases steady-state levels and half-lives of a subset of mature mitochondrial mRNAs MT-ND2, MT-ND3, MT-CYTB, MT-CO2, and MT-ATP8/6. Promotes MT-CO1 mRNA translation and increases mitochondrial complex IV assembly and activity (PubMed:27789713). Expression detected in spleen, thymus, testis, ovary, colon, heart, smooth muscle, kidney, brain, lung, liver and white adipose tissue with highest expression in liver and thyroid. RAP domain is required for FASTKD3 function in mRNA stability and translation. Belongs to the FAST kinase family.