Involved in the down-regulation of mitochondrial MT-ND3 mRNA levels which leads to decreased respiratory complex I abundance and activity. Preferentially localizes to mitochondrial RNA granules, platforms for post-transcriptional RNA modification and ribosome assembly. Expression detected in spleen, testis, colon, heart, smooth muscle, kidney, brain, lung, liver, brown and white adipose tissue with highest expression in heart and brown adipose tissue. The RAP domain is essential to regulate MT-ND3 mRNA levels. Belongs to the FAST kinase family.