The Siva-1 putative amphipathic helical region (SAH) is sufficient to bind to BCL-XL and sensitize cells to UV radiation induced apoptosis

The human Siva gene is localized to chromosome 14q32-33 and gives rise to the full-length predominant form, Siva-1 and a minor alternate form, Siva-2 that appears to lack the proapoptotic properties of Siva-1. Our recent work has shown that the missing region in Siva-2 encodes a unique twenty amino acid putative amphipathic helical region (SAH, residues 36-55 in Siva-1). Despite the fact that Siva-1 does not belong to the Bcl-2 Family, it specifically interacts with the anti-apoptotic protein Bcl-xL and sensitizes MCF7 breast Cancer cells expressing Bcl-xL to UV radiation induced Apoptosis. Deletion mutagenesis has mapped the necessary region to the SAH in Siva-1. In this paper we demonstrate that the SAH region in Siva-1 is sufficient to specifically interact with the anti-apoptotic members of the BCL2 family such as Bcl-xL and Bcl-2 but not its apoptotic member Bax. Using transient transfections and direct microinjection of synthetic SAH Peptides, we also demonstrate that the SAH region is sufficient to inhibit the Bcl-xL mediated cell survival and render MDA-MB-231 and MCF7 breast Cancer cells expressing Bcl-xL highly susceptible to UV radiation induced Apoptosis. The underlying mechanism of action of SAH mediated inhibition of Bcl-xL (and/or BCL2) cell survival appears to be due to loss of mitochondrial integrity as reflected in enhanced cytochrome c release leading to the activation of Caspase 9 and finally Caspase 3.