The COP9 signalosome (CSN) plays important roles in multifaceted cellular 
processes. Study has shown that inositol 1,3,4-trisphosphate 5/6 kinase (5/6 
kinase) interacts with CSN in mammalian cells. However, the biological function 
of the interaction still remains unknown. Here, we report that the Arabidopsis 
inositol 1,3,4-trisphosphate 5/6 kinase (AtItpk-1) is also associated with CSN 
and involved in photomorphogenesis under red light (RL) conditions, as 
demonstrated by co-immunoprecipitation of AtItpk-1 with CSN and characterization 
of the atitpk-1 mutants. Expression analysis showed that AtItpk-1 had the same 
sub-cellular localization and organ expression pattern as CSN. Furthermore, 
autophosphorylation analysis showed that AtItpk-1 has protein kinase activity. 
Under RL, the atitpk-1 mutants exhibited phenotype slightly similar with that of 
the csn mutants, indicating that 5/6 kinase might be involved in the same 
developmental pathway as CSN. This study suggests that AtItpk-1 may function as 
a protein kinase that is involved in photomorphogenesis possibly via interaction 
with COP9 signalosome under red light.