How cholera toxin breaches the ER membrane to trigger intoxication

Cholera toxin (CT) secreted by Vibrio cholerae is the virulence factor responsible for inducing massive secretory diarrhea that typifies cholera. To intoxicate intestinal epithelial cells, CT binds to the ganglioside GM1 receptor on the cell surface and is transported in a retrograde manner to the lumen of the endoplasmic reticulum (ER). Here, the toxic component of CT, called CTA1, peptide acts similar to a misfolded protein and co-opts the ER-associated degradation (ERAD) machinery to retro-translocate to the cytosol. The ERAD machinery normally disposes misfolded and misassembled ER proteins to the cytosol for ubiquitin-dependent proteasomal degradation. However, when CTA1 reaches the cytosol, it is not ubiquitinated, evades proteasomal destruction, and promotes a signaling cascade resulting in pathologic water secretion that leads to disease. While intense research has revealed CTA1’s action in the cytosol, how it reaches the cytosol from the ER remains mysterious. What steps and host factors are hijacked during this pathway? We intend to address these questions in this project by using a combination of classical biochemistry and cell biology, coupled with advanced imaging methods.

Project #1 

Project #2 

How polyomavirus penetrates the ER membrane to cause infection

Polyomaviruses (Pys) are non-enveloped DNA tumor viruses that cause a variety of human diseases, particularly in immunocompromised individuals. This family of viruses includes Merkel cell carcinoma by the human Merkel cell polyomavirus and progressive multifocal leukoencephalopathy by the human JC polyomavirus. To infect cells, Py binds to ganglioside receptors on the host cell surface and is internalized. The virus then traffics retrograde manner to the ER lumen where it hijacks cellular machineries to cross the ER membrane and reach the cytosol. From the cytosol, Py is further transported to the nucleus where transcription and replication of the viral DNA ensue, leading to lytic infection or cell transformation. How this virus penetrates the ER membrane to reach the cytosol, a decisive infection event, remains enigmatic. We will probe this mysterious process in this project by using a combination of biochemistry, cell biology, and microscopy strategies.

Tsai  Lab