The Arabidopsis homologue of an eIF3 complex subunit associates with the COP9 complex

The Arabidopsis COP9 complex is a multi-subunit repressor of photomorphogenesis which is conserved among multicellular organisms. Approximately 12 proteins copurify with the COP9 complex. Seven of these proteins are orthologues of subunits of the recently published mammalian COP9 complex. Four of the proteins show amino acid similarity to various subunits of the COP9 complex, eIF3 complex and 19S cap of the proteasome. We have studied one of these proteins in order to determine if it is a component of the COP9 complex. Arabidopsis p105 is highly similar to the p110 subunit of the human elF3. The p105 gene is induced during photomorphogenesis, and RNA and protein analysis reveal different tissue accumulation patterns. p105 is found in a large protein complex. p105 interacts in yeast with both COP9 and FUS6, two known components of the COP9 complex. Our results indicate that p105 is not a component of the COP9 core complex, though it may interact with components of the complex.