Identification of the Major Phosphorylation Site in Bcl-xL Induced by Microtubule Inhibitors and Analysis of Its Functional Significance

Overview

Journal J Biol Chem

Specialty Biochemistry

Date 2008 Nov 1

PMID 18974096

Citations 56

Authors

Meenakshi Upreti

Elena N Galitovskaya

Rong Chu

Alan J Tackett

David T Terrano

Susana Granell

Timothy C Chambers

Affiliations

Soon will be listed here.

Abstract

Vinblastine and other microtubule inhibitors used as antimitotic cancer drugs characteristically promote the phosphorylation of the key anti-apoptotic protein, Bcl-xL. However, putative sites of phosphorylation have been inferred based on potential recognition by JNK, and no direct biochemical analysis has been performed. In this study we used protein purification and mass spectrometry to identify Ser-62 as a single major site in vivo. Site-directed mutagenesis confirmed Ser-62 to be the site of Bcl-xL phosphorylation induced by several microtubule inhibitors tested. Vinblastine-treated cells overexpressing a Ser-62 --> Ala mutant showed highly significantly reduced apoptosis compared with cells expressing wild-type Bcl-xL. Co-immunoprecipitation revealed that phosphorylation caused wild-type Bcl-xL to release bound Bax, whereas phospho-defective Bcl-xL retained the ability to bind Bax. In contrast, phospho-mimic (Ser-62 --> Asp) Bcl-xL exhibited a reduced capacity to bind Bax. Functional tests were performed by transiently co-transfecting Bax in the context of different Bcl-xL mutants. Co-expression of wild-type or phospho-defective Bcl-xL counteracted the adverse effects of Bax expression on cell viability, whereas phospho-mimic Bcl-xL failed to provide the same level of protection against Bax. These studies suggest that Bcl-xL phosphorylation induced by microtubule inhibitors plays a key pro-apoptotic role at least in part by disabling the ability of Bcl-xL to bind Bax.

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