A novel inhibitor of cap-dependent translation initiation in yeast : p 20 competes with eIF 4 G for binding to eIF 4 E growth and proliferation of mammalian cells

growth and proliferation of mammalian cells. Increasing Michael Altmann1, Nicole Schmitz, the expression of eIF4E in rodent fibroblasts, either by Catherine Berset and Hans Trachsel cDNA transfection (Lazaris-Karatzas et al., 1990) or by Institute for Biochemistry and Molecular Biology, University of Bern, microinjecting the protein (Smith et al., 1990) stimulates Bühlstrasse 28, CH-3012 Bern, Switzerland cell growth and causes the cells to acquire a transformed 1Corresponding author phenotype. The transforming activity of high levels of eIF4E in mammalian cells may be explained by increased M.Altmann and N.Schmitz contributed equally to this work eIF4F activity stimulating the translation of mRNAs for growth promoting genes such as cyclin D1 (Rosenwald In the yeast Saccharomyces cerevisiae a small protein et al., 1993), ornithine decarboxylase (Shantz and Pegg, named p20 is found associated with translation initi1994) and c-myc (de Benedetti et al., 1994). Regulation of ation factor eIF4E, the mRNA cap-binding protein. cap-binding activity is achieved in several ways including We demonstrate here that p20 is a repressor of capphosphorylation of eIF4E, phosphorylation of a protein dependent translation initiation. p20 shows amino acid which inhibits the interaction of eIF4E with eIF4G (see sequence homology to a region of eIF4G, the large below) and by phosphorylation of eIF4G (Mader and subunit of the cap-binding protein complex eIF4F, Sonenberg, 1995). Whereas the mechanism by which which carries the binding site for eIF4E. Both, eIF4G phosphorylation of eIF4E and eIF4G affect cap-binding and p20 bind to eIF4E and compete with each other activity at the molecular level are poorly understood, we for binding to eIF4E. The eIF4E–p20 complex can learned more recently how eIF4E activity is regulated by bind to the cap structure and inhibit cap-dependent a group of proteins which interact with eIF4E, termed 4Ebut not cap-independent translation initiation: the BPs (4E-binding proteins) (Lin et al., 1994; Pause et al., translation of a mRNA with the 67 nucleotide Ω 1994). These proteins compete with eIF4G for interaction sequence of tobacco mosaic virus in its 5 untranslated with eIF4E, interfere in this way with the formation of region (which was previously shown to render translathe eIF4F complex and thereby inhibit cap-dependent tion cap-independent) is not inhibited by p20. Whereas translation (Haghighat et al., 1995). Their activity is the translation of the same mRNA lacking the Ω regulated by phosphorylation. In adipose cells treated with sequence is strongly inhibited by p20. Disruption of insulin, 4E-BPs become phosphorylated and dissociate CAF20, the gene encoding p20, stimulates the growth from eIF4E, thereby relieving translational inhibition of yeast cells, overexpression of p20 causes slower (Pause et al., 1994). The phosphorylation of 4E-BPs growth of yeast cells. These results show that p20 is appears to be under the control of the FRAP/RAFT signal a regulator of eIF4E activity which represses captransduction pathway since treatment of NIH 3T3 cells dependent initiation of translation by interfering with with rapamycin results in inhibition of 4E-BP1 phosphorylthe interaction of eIF4E with eIF4G, e.g. the formation ation (Beretta et al., 1996; von Manteuffel et al., 1996). of the eIF4F–complex. In S.cerevisiae the eIF4F complex is composed of the