The Characterization of Baxb, a Splice Variant of the Pro-Apoptotic Bax and Protein

Abstract

Baxa is a pro-apoptotic member of the Bcl-2 protein family that regulates a key point in the control of apoptosis. The bax RNA undergoes a complex pattern of RNA splicing, with eight splice isoforms known to date. The next most abundant isoform to Baxa is Baxp, which has a unique carboxyl-terminal sequence and consequently lacks the transmembrane domain of Baxa. This study characterized Baxp as part of a larger project aimed at deducing the role of cellular localization in Bax protein function. A transient transfection assay was designed to determine the cell death activity of a protein in adherent cells. Baxp induced cell death to a greater extent than Baxa when transiently expressed in NIH 3T3 cells. The levels of Baxp expression were always low, regardless of the cell type or transfection method used. Additionally, Baxp adopted a conformation in which amino acids 13 to 19 are accessible to the monoclonal antibody 6A7. In Baxa this epitope is only exposed in the membrane-bound activated conformation that is associated with accessibility of the BH3 domain of Baxa that mediates protein-protein interactions among Bcl-2 family members. Although the mechanism remains elusive, Baxp was identified as a potent inducer of cell death.