Coilin is a marker protein for the Cajal body, a subnuclear domain acting as a 
site for assembly and maturation of nuclear RNA-protein complexes. Using a yeast 
two-hybrid screen to identify coilin-interacting proteins, we have identified 
hCINAP (human coilin interacting nuclear ATPase protein), a nuclear factor of 
172 amino acids with a P-loop nucleotide binding motif and ATPase activity. The 
hCINAP protein sequence is highly conserved across its full-length from human to 
plants and yeast and is ubiquitously expressed in all human tissues and cell 
lines tested. The yeast orthologue of CINAP is a single copy, essential gene. 
Tagged hCINAP is present in complexes containing coilin in mammalian cells and 
recombinant, Escherichia coli expressed hCINAP binds directly to coilin in 
vitro. The 214 carboxyl-terminal residues of coilin appear essential for the 
interaction with hCINAP. Both immunofluorescence and fluorescent protein tagging 
show that hCINAP is specifically nuclear and distributed in a widespread, 
diffuse nucleoplasmic pattern, excluding nucleoli, with some concentration also 
in Cajal bodies. Overexpression of hCINAP in HeLa cells results in a decrease in 
the average number of Cajal bodies per nucleus, consistent with it affecting 
either the stability of Cajal bodies and/or their rate of assembly. The hCINAP 
mRNA is an alternatively spliced transcript from the TAF9 locus, which encodes 
the basal transcription factor subunit TAFIID32. However, hCINAP and TAFIID32 
mRNAs are translated from different ATG codons and use distinct reading frames, 
resulting in them having no identity in their respective protein sequences.