Human coilin interacting nuclear ATPase protein (hCINAP) directly interacts with 
coilin, a marker protein of Cajal Bodies (CBs), nuclear organelles involved in 
the maturation of small nuclear ribonucleoproteins UsnRNPs and snoRNPs. hCINAP 
has previously been designated as an adenylate kinase (AK6), but is very 
atypical as it exhibits unusually broad substrate specificity, structural 
features characteristic of ATPase/GTPase proteins (Walker motifs A and B) and 
also intrinsic ATPase activity. Despite its intriguing structure, unique 
properties and cellular localization, the enzymatic mechanism and biological 
function of hCINAP have remained poorly characterized. Here, we offer the first 
high-resolution structure of hCINAP in complex with the substrate ADP (and 
dADP), the structure of hCINAP with a sulfate ion bound at the AMP binding site, 
and the structure of the ternary complex hCINAP-Mg(2+) ADP-Pi. Induced fit 
docking calculations are used to predict the structure of the hCINAP-Mg(2+) 
ATP-AMP ternary complex. Structural analysis suggested a functional role for 
His79 in the Walker B motif. Kinetic analysis of mutant hCINAP-H79G indicates 
that His79 affects both AK and ATPase catalytic efficiency and induces homodimer 
formation. Finally, we show that in vivo expression of hCINAP-H79G in human 
cells is toxic and drastically deregulates the number and appearance of CBs in 
the cell nucleus. Our findings suggest that hCINAP may not simply regulate 
nucleotide homeostasis, but may have broader functionality, including control of 
CB assembly and disassembly in the nucleus of human cells.