The pleiotropic constitutive photomorphogenic/deetiolated/fusca (cop/det/fus) 
mutants of Arabidopsis exhibit features of light-grown seedlings when grown in 
the dark. Cloning and biochemical analysis of COP9 have revealed that it is a 
component of a multiprotein complex, the COP9 signalosome (previously known as 
the COP9 complex). Here, we compare the immunoaffinity and the biochemical 
purification of the COP9 signalosome from cauliflower and confirm its 
eight-subunit composition. Molecular cloning of subunit 4 of the complex 
revealed that it is a proteasome-COP9 complex-eIF3 domain protein encoded by a 
gene that maps to chromosome 5, near the chromosomal location of the cop8 and 
fus4 mutations. Genetic complementation tests showed that the cop8 and fus4 
mutations define the same locus, now designated as COP8. Molecular analysis of 
the subunit 4-encoding gene in both cop8 and fus4 mutants identified specific 
molecular lesions, and overexpression of the subunit 4 cDNA in a cop8 mutant 
background resulted in complete rescue of the mutant phenotype. Thus, we 
conclude that COP8 encodes subunit 4 of the COP9 signalosome. Examination of 
possible molecular interactions by using the yeast two-hybrid assay indicated 
that COP8 is capable of strong self-association as well as interaction with 
COP9, FUS6/COP11, FUS5, and Arabidopsis JAB1 homolog 1, the latter four proteins 
being previously defined subunits of the Arabidopsis COP9 signalosome. A 
comparative sequence analysis indicated that COP8 is highly conserved among 
multicellular eukaryotes and is also similar to a subunit of the 19S regulatory 
particle of the 26S proteasome.