Inactivation of mRNA cap-binding protein complex in Drosophila melanogaster embryos under heat shock.

We have studied the role of Drosophila 35-kDa cap-binding protein (CBP) and CBP complex in the mechanism of messenger RNA discrimination established in heat-shocked Drosophila embryos. Drosophila 35-kDa CBP is functionally equivalent to the mammalian eucaryotic initiation factor (eIF)-4E and CBP complex, which includes eIF-4E, might be the counterpart of mammalian eIF-4F. By using anti-eIF-4E antibodies, we found that although translation of the bulk of normal messengers in Drosophila lysates was very dependent on eIF-4E, the mRNAs for the heat shock proteins (hsps) (particularly hsp70 mRNA and with the exception of hsp83 mRNA) were translated almost independently of this factor, suggesting that they may have unstructured leaders. Accordingly, hsp70 mRNA and, to a lesser extent, the mRNAs for the small hsps were found to be more resistant to inhibition by K+ than normal and hsp83 mRNAs. Moreover, Drosophila CBP complex was able to rescue partial but specifically the synthesis of normal proteins when added to a lysate from heat-shocked embryos. However, no significant effect was obtained by Drosophila eIF-4E or eIF-2. Consistent with these results, we found a great decrease in the amount of the CBP complex purified from heat-shocked embryos as compared with normal ones, whereas the amounts of free eIF-4E purified from either source were similar. Together, the above results suggest that some modification leading to the disruption of Drosophila CBP complex may account, at least to some extent, for the mRNA discrimination established in heat-shocked Drosophila embryos.