title: Bcl-x(S) anatagonizes the protective effects of Bcl-x(L).
abstract: 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.
authors.affiliation: Gwen Knapp Center for Lupus and Immunology Research, The University of Chicago, Illinois 60637, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Minn
authors.forename: L H
authors.initials: LH
authors.lastname: Boise
authors.forename: C B
authors.initials: CB
authors.lastname: Thompson
mesh_terms: D017398:Alternative Splicing; D000818:Animals; D017209:Apoptosis / Q000235:genetics / Q000502:physiology; D001483:Base Sequence; D002460:Cell Line; D017931:DNA Primers / Q000235:genetics; D018432:Drug Resistance, Multiple / Q000235:genetics / Q000502:physiology; D000939:Epitopes / Q000235:genetics; D066298:In Vitro Techniques; D051379:Mice; D008969:Molecular Sequence Data; D009842:Oligopeptides; D010455:Peptides / Q000502:physiology; D010641:Phenotype; D011233:Precipitin Tests; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000276:immunology / Q000502:physiology*; D019253:Proto-Oncogene Proteins c-bcl-2; D014162:Transfection; D051028:bcl-2-Associated X Protein; D051020:bcl-X Protein
chemical_list: C494827:Bax protein, mouse; C494812:Bcl2l1 protein, mouse; D017931:DNA Primers; D000939:Epitopes; D009842:Oligopeptides; D010455:Peptides; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D051028:bcl-2-Associated X Protein; D051020:bcl-X Protein; C085706:FLAG peptide