Structure and expression of human mitochondrial adenylate kinase targeted to the mitochondrial matrix

The previously isolated cDNA encoding human adenylate kinase (AK) isozyme 3 was recently renamed AK4. Consequently, human AK3 cDNA remains to be identified and we have little information about the functional relationship between human AK3 and AK4. In pursuit of the physiological roles of both the AK3 and AK4 proteins, we first isolated an authentic human AK3 cDNA and compared their expression. Nucleotide sequencing revealed that the cDNA encoded a 227-amino-acid protein, with a deduced molecular mass of 25.6 kDa, that shares greater homology with the AK3 cDNAs isolated from bovine and rat than that from human. We named the isolated cDNA AK3. Northern-blot analysis revealed that AK3 mRNA was present in all tissues examined, and was highly expressed in heart, skeletal muscle and liver, moderately expressed in pancreas and kidney, and weakly expressed in placenta, brain and lung. On the other hand, we found that human AK4 mRNA was highly expressed in kidney, moderately expressed in heart and liver and weakly expressed in brain. Western-blot analysis demonstrated expression profiles of AK3 and AK4 that were similar to their mRNA expression patterns in each tissue. Over expression of AK3, but not AK4, in both Escherichia coli CV2, a temperature-sensitive AK mutant, and a human embryonic kidney-derived cell line, HEK-293, not only produced significant GTP:AMP phosphotransferase (AK3) activity, but also complemented the CV2 cells at 42 degrees C. Subcellular and submitochondrial fractionation analysis demonstrated that both AK3 and AK4 are localized in the mitochondrial matrix.