Arabidopsis FUSCA5 encodes a novel phosphoprotein that is a component of the COP9 complex

The COP9 complex is a regulator essential for repression of light-mediated development in Arabidopsis. Using partial amino acid sequence data generate d from purified COP9 complexes, we cloned the Arabidopsis cDNA encoding the 27-kD subunit of the COP9 complex and showed that it is encoded by the pre viously identified FUSCA5 (FUS5) locus. fus5 mutants exhibit constitutive p hotomorphogenic phenotypes similar to those of cop9 and fus6. Point mutatio ns in FUSE that led to a loss of FUS5 protein were detected in four fus5 al lelic strains. FUSE contains the PCI/PINT and mitogen-activated protein kin ase kinase activation loop motifs and is highly conserved with the mammalia n COP9 complex subunit 7 and the Aspergillus nidulans AcoB proteins. FUSE i s present in both complex and monomeric forms. In the COP9 complex, FUS5 ma y interact directly with FUS6 and COP9. Mutations in FUS6 and COP9 result i n a shift in the electrophoretic mobility of FUS5. This shift can be mimick ed by in vitro phosphorylation of FUS5 by plant extracts. These findings fu rther support the hypothesis that the COP9 complex is a central and common regulator that may interact with multiple signaling pathways.