Identification of the goldfish 20S proteasome β6 subunit bound to nuclear matrix

Proteasomes are large, multisubunit particles that act as the proteolytic machinery for most of the regulated intracellular protein breakdown in eukaryotic cells. Proteasomes are present in both the nucleus and cytoplasm. When we analyzed the molecular composition of protein constituents of the nuclear matrix preparation of goldfish oocytes by two‐dimensional polyacrylamide gel electrophoresis followed by sequence analysis, we found a 26 kDa spot identical in amino acid sequence to the β6 subunits of the 20S proteasome. No spot of other subunits of 20S proteasome was detected. Here we describe the cloning, sequencing and expression analysis of Carassius auratus, β6_ca, which encodes one of the proteasome β subunits from goldfish ovary. From the screening of an ovarian cDNA library, two types of cDNA were obtained, one 941 bp and the other 884 bp long. The deduced amino acid sequences comprise 239 and 238 residues, respectively. These deduced amino acid sequences are highly homologous to those of β6 subunits of other vertebrates. Immunoblot analysis of nuclear matrix using anti‐proteasome antibodies showed only a spot of β6_ca. These results suggest that the β6 subunit of the goldfish 20S proteasome, β6_ca, is responsible for anchoring proteasomes in the nucleus.