bcl -xl

bcl-x is a bcl-2-related gene that can function as a regulator of programmed cell death (apoptosis) independent of bcl-2. Alternative splicing results in two distinct bcl-x mRNAs. The larger mRNA gives rise to a protein product, bcl-xl, which is similar in size and predicted structure to bcl-2 (1). The smaller mRNA gives rise to bcl-xS. bcl-x immunoreactivity has been detected in a wide variety of cell types and the protein is typically present in the cytosol in association with the mitochondrial periphery, a property shared with bcl-2 however membrane bound forms of bcl-x have been demonstrated in thymocytes (2-4). Following the induction of apoptosis all of the bcl-x protein shifts to the membrane form (2). Of the two isoforms of bcl-x, the long (bcl-xl) is the most abundant mRNA species expressed in embryonic and adult tissues and most likely differs from bcl-2 in its regulatory activity on cell differentiation through controlled tissue specific expression (1,3). Like its homolog bcl-2, bcl-x undergoes phosphorylation, a modification that requires that a specific 60 amino acid loop region be intact, which in turn appears to regulate activity (5,6). Structurally, based on 3D-structure analysis, bcl-x forms pH sensitive cation-selective ion channels in membranes a property shared with the pore forming domains of several bacterial toxins (7). Bcl-xl has been shown to modify the cellÕs response to oxidants, to participate in resistance to chemotherapeutic agents and radiation, and to play a key role in the development of the developing CNS (8-10).