Aβ (amyloid beta peptide) is an important contributor to Alzheimer’s disease (AD). We modeled Aβ toxicity in yeast by directing the peptide to the secretory pathway. A genome-wide screen for toxicity modifiers identified the yeast homolog of phosphatidylinositol binding clathrin assembly protein (PICALM) and other endocytic factors connected to AD whose relationship to Aβ was previously unknown. The factors identified in yeast modified Aβ toxicity in glutamatergic neurons of Caenorhabditis elegans and in primary rat cortical neurons. In yeast, Aβ impaired the endocytic trafficking of a plasma membrane receptor, which was ameliorated by endocytic pathway factors identified in the yeast screen. These links between Aβ, endocytosis, and human AD risk factors can be ascertained using yeast as a model system.

We have addressed the mechanism by which proteins are posttranslationally transported across the membrane of the yeast endoplasmic reticulum (ER). We demonstrate that BiP (Kar2p), a member of the Hsp70 family resident in the ER lumen, acts as a molecular ratchet during translocation of the secretory protein prepro-alpha factor through the channel formed by the Sec complex. Multiple BiP molecules associate with each translocation substrate following interaction with the J domain of the Sec63p component of the Sec complex. Bound BiP minimizes passive backward movements of the substrate through the channel, and BiP's subsequent dissociation results in a free polypeptide in the ER lumen. Antibodies against the substrate can replace BiP, indicating that a Brownian ratchet is sufficient to achieve translocation.