Plant 21D7 protein, a nuclear antigen associated with cell division, is a component of the 26S proteasome

Previously, we cloned a carrot (Daucus carota L.) cDNA encoding a 45-kD protein, 21D7, located in the nuclei of proliferating cells. The 21D7 protein is similar to the partial sequence of a regulatory subunit of the bovine 26S proteasome, p58 (G. DeMartino, C.R. Moomaw, O.P. Zagnitko, R.J. Proske, M. Chu-Ping, S.J. Afendis, J.C. Swaffield, C.A. Slaughter [1994] J Biol Chem 269: 20878-20884) and to the deduced sequence encoded by the Saccharomyces cerevisiae gene SUN2 (M. Kawamura, K. Kominami, J. Takeuchi, A. Toh-e [1996] Mol Gen Genet 251: [146-152]). In our work, the expression of plant 21D7 cDNA rescued the yeast sun2 mutant. Fractionation of carrot and spinach (Spinacia oleracea L.) crude extracts showed that the 21D7 protein sedimented with the active 26S proteasomes. The cessation of cell proliferation in carrot suspensions at the stationary phase caused 26S proteasome dissociation and, correspondingly, the 21D7 protein sedimented together with the free regulatory complexes of the 26s proteasomes. Large-scale purification of carrot 26s proteasomes resulted in co-isolation of the 21D7 protein. Polyacrylamide gel electrophoresis under nondenaturing conditions showed that the 21D7 protein had the same mobility as the 26S proteasome and that proteasome dissociation changed the mobility of the 21D7 protein accordingly. We conclude that the 21D7 protein is a subunit of the plant 26S proteasome and that it probably belongs to the proteasome regulatory complex.