A novel "active" form of proteasomes from Xenopus laevis ovary cytosol

The cytosol fraction prepared from Xenopus laevis ovaries by one-step ultracentrifugation catalyzed the hydrolysis of succinyl-leucyl-leucyl-valyl-tyrosine-4-methylcoumaryl-7-amide with or without SDS, an activator of 20S latent proteasomes. Both activities were lost on immunodepletion with antibodies against 20S proteasome. Storage of the cytosol at 4 degrees C led to abolition of the SDS-independent activity, but not the SDS-dependent activity. Upon DEAE-cellulose chromatography, the enzyme catalyzing the SDS-independent activity could be separated from that responsible for the SDS-dependent activity. These results indicate that the ovary cytosol contains a large proportion of a novel, "active" form of proteasomes which does not require SDS but is unstable and is readily converted to a latent form which requires SDS. ATP is known to stabilize 26S protease complex, but ATP reduced SDS-independent activity. It seems that the "active" form of proteasomes (molecular weight: about 1000-kDa) is different from 26S protease complex.