Plays a role in processing of mitochondrial RNA precursors and in stabilization of a subset of mature mitochondrial RNA species, such as MT-CO1, MT-CO2, MT-CYB, MT-CO3, MT-ND3, MT-ND5 and MT-ATP8/6. May play a role in cell cycle progression (PubMed:9383053). Ubiquitously expressed (PubMed:12759187). Expression detected in spleen, thymus, testis, ovary, colon, heart, smooth muscle, kidney, brain, lung, liver and white adipose tissue with highest expression in smooth muscle (PubMed:20869947). RAP domain is required for TBRG4 function in mRNA stability and translation. Belongs to the FAST kinase family.