Bcl-x deamidation is regulated by multiple ion transporters and is intramolecularly catalyzed.

In susceptible tumor cells, DNA-damaging antineoplastic agents induce an increase in intracellular pH during the premitochondrial stage of apoptosis. The rate of nonenzymatic deamidation of two asparagines in the anti-apoptotic protein Bcl-x is accelerated by this increase in pH. Deamidation of these asparagines is a signal for the degradation of Bcl-x, which is a component of the apoptotic response to DNA damage. It has previously been shown that the increase in pH is mediated by the ion transporter Na/H exchanger 1 in some cells. Here we demonstrate that one or more additional ion transporters also have a role in the regulation of Bcl-x deamidation in at least some tumor cell lines and fibroblasts. As a second, independent finding, we report that there are histidines in close proximity to the Bcl-x deamidation sites that are highly conserved in land-dwelling species and we present evidence that deamidation of human Bcl-x is intramolecularly catalyzed in a manner that is dependent upon these histidines. Further, we present evidence that these histidines act as a pH-sensitive switch that enhances the effect of the increase in pH on the rate of Bcl-x deamidation. The conservation of such histidines implies that human Bcl-x is in essence "designed" to be deamidated, which provides further evidence that deamidation serves as a bona fide regulatory post-translational modification of Bcl-x.