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 14q3233 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 antiapoptotic 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.