Yeast Bax Inhibitor, Bxi1p, Is an ER-Localized Protein That Links the Unfolded Protein Response and Programmed Cell Death in Saccharomyces cerevisiae

James Cebulski,Nathan Pinches,Joshua Malouin,Vincent Cascio,Nicanor Austriaco,Matt Kaeberlein

doi:10.1371/journal.pone.0020882

Abstract

Bax inhibitor-1 (BI-1) is an anti-apoptotic gene whose expression is upregulated in a wide range of human cancers. Studies in both mammalian and plant cells suggest that the BI-1 protein resides in the endoplasmic reticulum and is involved in the unfolded protein response (UPR) that is triggered by ER stress. It is thought to act via a mechanism involving altered calcium dynamics. In this paper, we provide evidence that the Saccharomyces cerevisiae protein encoded by the open reading frame, YNL305C, is a bona fide homolog for BI-1. First, we confirm that yeast cells from two different strain backgrounds lacking YNL305C, which we have renamed BXI1, are more sensitive to heat-shock induced cell death than wildtype controls even though they have indistinguishable growth rates at 30°C. They are also more susceptible both to ethanol-induced and to glucose-induced programmed cell death. Significantly, we show that Bxi1p-GFP colocalizes with the ER localized protein Sec63p-RFP. We have also discovered that Δbxi1 cells are not only more sensitive to drugs that induce ER stress, but also have a decreased unfolded protein response as measured with a UPRE-lacZ reporter. Finally, we have discovered that deleting BXI1 diminishes the calcium signaling response in response to the accumulation of unfolded proteins in the ER as measured by a calcineurin-dependent CDRE-lacZ reporter. In toto, our data suggests that the Bxi1p, like its metazoan homologs, is an ER-localized protein that links the unfolded protein response and programmed cell death.

Highlights

We have discovered that Dbxi1 cells are more sensitive to drugs that induce ER stress, and have a decreased unfolded protein response as measured with a UPRE-lacZ reporter
[18] In contrast, there is conflicting data regarding the function of the putative yeast Bax inhibitor-1 homolog encoded by the Saccharomyces cerevisiae open reading frame, YNL305C
Bax inhibitor-1 is a highly conserved mammalian protein with homologs found in Arabidopsis thaliana, Drosophila melanogaster, Figure 1

Summary

Introduction

Bax inhibitor-1 (BI-1) was first cloned in mammals from adult testes of different species, as the testis enhanced gene transcript (TEGT). [1,2,3,4] its anti-apoptotic function was identified several years later through a functional yeast screen designed to select for human cDNAs that inhibit BAX induced apoptosis. [5] Importantly, BI-1 suppressed apoptosis induced by BAX in human 293 kidney cells. [6]The anti-apoptotic function of BI-1 contributes to the development of cancer: BI-1 expression is upregulated in human breast cancer, prostate cancer, brain tumors, large cell lymphoma, and cervical cancer. [7,8,9,10,11,12,13] In contrast, down-regulation of the gene is associated with the progression of chronic liver disease.[14]. We confirm that yeast cells from two different strain backgrounds lacking YNL305C, which we have renamed BXI1, are more sensitive to heat-shock induced cell death than wildtype controls even though they have indistinguishable growth rates at 30uC.

Results

Conclusion