Fig. 4. Energy support relays for nucleocytoplasmic communication. Mitochondria clustered around the nucleus generate the majority of ATP required for nuclear processes. Export of ATP from the mitochondrial intracristal space is facilitated by near-equilibrium reactions catalyzed by mitochondrial isoforms of adenylate kinase (AK₂), creatine kinase (Mi-CK) and nucleoside diphosphate kinase (NDPK). Subsequently, high-energy phosphoryls are navigated through the diffusionally restricted perinuclear space to ATP consumption sites at the nuclear envelope and inside the nucleus by cytosolic and nuclear isoforms of AK, CK and NDPK. Interaction and complementation between these systems secure proper nucleotide ratios at and across the nuclear envelope, sustaining the high energy of ATP and GTP hydrolysis. For other abbreviations, see Fig. 3.