The cellular level of PR500, a protein complex related to the 19S regulatory particle of the proteasome, is regulated in response to stresses in plants

In Arabidopsis seedlings and cauliflower florets, Rpn6 (a proteasome non-AT Pase regulatory subunit) was found in two distinct protein complexes of sim ilar to 800 and 500 kDa, respectively. The large complex likely represents the proteasome 19S regulator particle (RP) because it displays the expected subunit composition and all characteristics. The small complex, designated PR500, shares at least three subunits with the "lid" subcomplex of 19S RP and is loosely associated with an hsp70 protein. In Arabidopsis COP9 signal osome mutants, PR500 was specifically absent or reduced to an extent that c orrelates with the severity of the mutations. Furthermore, PR500 was also d iminished in response to potential protein-misfolding stresses caused by th e heat shock and canavanine treatment. Immunofluorescence studies suggest t hat PR500 has a distinct localization pattern and is enriched in specific n uclear foci. We propose that PR500 may be evolved in higher plants to cope with the frequently encountered environmental stresses.