A gain-of-function phenotype conferred by over-expression of functional subunits of the COP9 signalosome in Arabidopsis

The COP9 signalosome is a conserved cellular regulator present in diverse o rganisms. To understand the structural and functional relationship of the C OP9 signalosome with its subunits, we expressed in wild-type and mutant Ara bidopsis backgrounds two orthologues of subunit 1, rice FUS6 (rFUS6) and hu man GPS1, and Arabidopsis subunit 8 (COP9). In Arabidopsis, rFUS6 can funct ionally replace Arabidopsis endogenous FUS6 to form the COP9 signalosome co mplex and rescue the null fus6-1 mutant phenotype. Moreover, light-grown rF US6 over-expression seedlings displayed longer hypocotyls and reduced antho cyanin accumulation in comparison to wild-type seedlings, which is opposite to the fus6/cop11 mutant phenotype. The long-hypocotyl phenotype was also observed in transgenic seedlings over-expressing Arabidopsis COP9. This fin ding indicates that over-expression of a functional subunit 1 or subunit 8 of the COP9 signalosome confers a gain-of-function phenotype relative to th e complex. Human GPS1, when expressed in the fus6-1 null mutant of Arabidop sis, can assemble into a chimeric COP9 signalosome at low efficiency, demon strating the structural conservation of the complexes between human and Ara bidopsis. This low-abundancy chimeric complex is insufficient to fully resc ue the mutant but is able to attenuate the mutant severity.