Molecular and biochemical characterization of a proteasome subunit from rice and carrot cells

Proteasomes function mainly in the ATP-dependent degradation of proteins that have been conjugated with ubiquitin. We isolated a cDNA clone for a rice protein that exhibited high homology to subunit C2 of proteasomes. Southern blot analysis revealed that the corresponding gene was present as a single copy in the rice genome. After fractionation of a crude extract from cultured cells, a 35-kDa protein that cross-reacted with antibodies against the C2 subunit was recovered in the peak fraction of both 20 S and 26 S complexes. The same antibodies cross-reacted with two proteins in seedlings, one of which was the same as that detected in cultured cells. The level of the protein was reduced in roots under conditions of high salinity. The 35-kDa protein was not detected in the nuclei of rice or carrot cells. However, during somatic embryogenesis of carrot cells, the C2 subunit was found in the nucleus at the globular stage, and it gradually disappeared in the period from the heart to the torpedo stage. Cells at the globular stage are proliferating rapidly, thus, it is possible that proteasomes are associated with the proliferation of plant cells.