Accumulation of misfolded protein aggregates leads to the formation of russell body-like dilated endoplasmic reticulum in yeast.

RNAP-1, an aspartic proteinase from a filamentous fungus Rhizopus niveus, is secreted very efficiently in Saccharomyces cerevisiae. It is synthesized first as a precursor form with signal sequence and prosequence in its amino-terminus. Our previous study indicated that the prosequence of RNAP-I had important roles in its correct folding and secretion in yeast, and that a prosequence-deleted derivative of RNAP-I, delta pro, was not secreted but was retained and degraded in the yeast endoplasmic reticulum (ER). In the present study, we show that the accumulation of delta pro in the yeast ER caused elevated synthesis of ER resident chaperones, indicating that delta pro is recognized as an unfolded protein species in the ER. Our biochemical data demonstrated that delta pro formed aggregates which contained BiP, but not protein disulfide isomerase (PDI), in the ER. Immunoelectron microscopical analysis revealed that the delta pro aggregates were indeed visible as electron-dense regions in the ER and nuclear envelope. Such 'chaperone-associated misfolded protein bodies' were observed for the first time in yeast. Morphologies of the ER and nucleus were drastically altered by the accumulation of the delta pro aggregates. The ER lost its flat cisternal shape; the ER lumen extended aberrantly and the ER membrane irregularly proliferated. The misfolded delta pro proteins are probably sorted from the ordinary ER lumen to form the aggregates so that the ER function would not be grossly impaired, and the dilated ER may represent an ER subcompartment where the delta pro aggregates are degraded.