P27BBP/eIF6 acts as an anti-apoptotic factor upstream of Bcl-2 during Xenopus laevis development

Abstract

p27(BBP)/eIF6 (beta4-binding protein/eukaryotic initiation factor 6) regulates the joining of 40S and 60S ribosomal subunits, on receptor for activated C kinase 1 binding and protein kinase C phosphorylation in serine 235. In Xenopus, p27(BBP)/eIF6 is abundantly expressed in the majority of the embryonic anlagen. Although p27(BBP)/eIF6 abundance may be required for a general regulation of protein synthesis, our data suggest that p27(BBP)/eIF6 may target the translation of specific mRNAs. We injected Xp27(BBP)/eIF6 mRNA in one blastomere of two-cell-stage embryos and obtained a bent phenotype, the curvature being lateral with respect to the embryo antero-posterior axis. The injected side had fewer apoptotic cells than the uninjected side, whereas cell proliferation appeared unaffected. Accordingly, in Xp27(BBP)/eIF6 morphants, endogenous apoptosis increased. Injection of Xp27(BBP)/eIF6 point mutants indicated that the anti-apoptotic action of Xp27(BBP)/eIF6 requires the conserved S235. The bent phenotype was also obtained with B-cell lymphoma gene-2 (Bcl-2) overexpression and was rescued by Bcl-2-associated X protein (Bax)/Xp27(BBP)/eIF6 co-injection. In addition, embryos overexpressing Xp27(BBP)/eIF6 had a higher amount of Bcl-2 and an unchanged amount of Bax with respect to controls. In Xp27(BBP)/eIF6 morphants, Bcl-2 levels were unaffected and Bax levels were higher than in the controls. Thus, we propose that Xp27(BBP)/eIF6 is part of a mechanism acting on the specific translation of messengers regulating cell survival. In particular, we suggest that Xp27(BBP)/eIF6 may regulate the translation of factors upstream of Bcl-2/Bax.