Functional analysis of EID1, an F-box protein involved in phytochrome A-dependent light signal transduction.

Empfindlicher im Dunkelroten Licht 1 (EID1) is an F-box protein that functions as a negative regulator in phytochrome A (phyA)-specific light signalling. F-box proteins are components of SCF ubiquitin ligase complexes that target proteins for degradation in the proteasome. Here we present further characterization of EID1 at the expression level, and show that it regulates photomorphogenesis in seedlings, rosette leaf development and flowering. Data on transcript expression patterns indicate that EID1 is expressed during all stages of Arabidopsis development and exhibits no light response. Microscope studies demonstrate that EID1 is localized to the nucleus, where it can form speckles under continuous far-red light that resemble clastosomes. To characterize the composition and formation of SCF(EID1) complexes further, we used two-hybrid and bridge assays in yeast and in planta. EID1 interacts specifically with several Arabidopsis Skp1-like (ASK) proteins and Cullin1 to form stable dimeric and trimeric complexes. Our results support a two-step association process in which the F-box protein binds first to the ASK adaptor, forming a unit which then associates with the catalytic core of the SCF complex. Finally, our data indicate that the EID1 target interaction domain is composed of two independent modules.