GENETIC AND MOLECULAR ANALYSIS OF AN ALLELIC SERIES OF COP1 MUTANTS SUGGESTS FUNCTIONAL ROLES FOR THE MULTIPLE PROTEIN DOMAINS

The Arabidopsis protein COP1, encoded by the CONSTITUTIVE PHOTOMORPHOG ENIC locus 1, is an essential regulatory molecule that plays a role in the repression of photomorphogenic development in darkness and in the ability of light-grown plants to respond to photoperiod, end-of-day f ar-red treatment, and ratio of red/far-red light. The COP1 protein con tains three recognizable structural domains: starting from the N termi nus, they are the zinc binding motif, the putative coiled-coil region, and the domain with multiple WD-40 repeats homologous to the P subuni t of trimeric G-proteins (G(beta)) To understand the functional implic ations of these structural motifs, 17 recessive mutations of the COP1 gene have been isolated based on their constitutive photomorphogenic s eedling development in darkness. These mutations define three phenotyp ic classes: weak, strong, and lethal. The mutations that fall into the lethal class are possible null mutations of COP1. Molecular analysis of the nine mutant alleles that accumulated mutated forms of COP1 prot ein revealed that disruption of the G(beta)-protein homology domain or removal of the very C-terminal 56 amino acids are both deleterious to COP1 function. In-frame deletions or insertions of short amino acid s tretches between the putative coiled-coil and G(beta)-protein homology domains strongly compromised COP1 function. However, a mutation resul ting in a COP1 protein with only the N-terminal 282 amino acids, inclu ding both the zinc binding and the coiled-coil domains, produced a wea k phenotypic defect. These results indicated that the N-terminal half of COP1 alone retains some activity and a disrupted C-terminal domain masks this remaining activity.