Minn A J, Boise L H, Thompson C B
Gwen Knapp Center for Lupus and Immunology Research, The University of Chicago, Illinois 60637, USA.
J Biol Chem. 1996 Mar 15;271(11):6306-12. doi: 10.1074/jbc.271.11.6306.
Bcl-x, a member of the Bcl-2 family, has two alternatively spliced forms, Bcl-x(L) and Bcl-x(S). Bcl-x(L), like Bcl-2, is able to protect cells from a wide variety of apoptotic stimuli. Bcl-x(S), as a result of alternative splicing, lacks 63 amino acids that comprise the region of greatest amino acid identity between Bcl-x(L) and Bcl-2. These amino acids contain the highly conserved BH1 and BH2 regions, which have been used to define the Bcl-2 family. We show that both Bel-x(L) and Bcl-x(S) are able to regulate cell survival in a dose-dependent fashion. Bcl-x(L) is able to increase the cellular apoptotic threshold and is able to form stable complexes with Bax both in vitro and in vivo. In contrast, Bcl-x(S) can effectively inhibit the protective effects of Bcl-x(L) following growth factor withdrawal and chemotherapeutic drug treatment. However, compared with Bax, Bcl-x(S) binds to Bcl-x(L) weakly when assessed by in vitro binding assays. Coimmunoprecipitation from mammalian cells demonstrates that Bcl-x(S) does not show an observable ability to form heterodimers with other Bcl-2 family members. In addition, overexpression of Bel-x(S) does not alter the ability of Bax to heterodimerize with Bcl-x(L) in vivo. Thus, Bcl-x(S) does not appear to function by competitively disrupting the formation of dimers composed of other Bcl-2 family members. This suggests that Bcl-x(S) can enhance cellular sensitivity to apoptosis via a mechanism of action distinct from other Bc1-2 family members that promote apoptosis.
Bcl-x是Bcl-2家族的成员之一，有两种选择性剪接形式，即Bcl-x(L)和Bcl-x(S)。与Bcl-2一样，Bcl-x(L)能够保护细胞免受多种凋亡刺激。由于选择性剪接，Bcl-x(S)缺少63个氨基酸，这些氨基酸构成了Bcl-x(L)和Bcl-2之间氨基酸一致性最高的区域。这些氨基酸包含高度保守的BH1和BH2区域，它们已被用于定义Bcl-2家族。我们发现，Bcl-x(L)和Bcl-x(S)都能够以剂量依赖的方式调节细胞存活。Bcl-x(L)能够提高细胞凋亡阈值，并且在体外和体内都能够与Bax形成稳定的复合物。相比之下，在生长因子撤除和化疗药物处理后，Bcl-x(S)能够有效抑制Bcl-x(L)的保护作用。然而，通过体外结合试验评估时，与Bax相比，Bcl-x(S)与Bcl-x(L)的结合较弱。从哺乳动物细胞中进行的共免疫沉淀表明，Bcl-x(S)没有显示出与其他Bcl-2家族成员形成异二聚体的可观察到的能力。此外，Bcl-x(S)的过表达在体内并没有改变Bax与Bcl-x(L)形成异二聚体的能力。因此，Bcl-x(S)似乎不是通过竞争性破坏由其他Bcl-2家族成员组成的二聚体的形成来发挥作用的。这表明，Bcl-x(S)可以通过一种不同于其他促进凋亡的Bcl-2家族成员的作用机制来增强细胞对凋亡的敏感性。
