In eukaryotes, eukaryotic translation initiation factor 4G (eIF4G) plays a central role in the recruitment of ribosomes to the mRNA 5′ end and is therefore critical for the regulation of protein synthesis (14
). Two homologues of eIF4G, eIF4GI and eIF4GII, have been cloned (15
). Although they differ in various respects, both homologues clearly function in translation initiation. The most thoroughly studied of these is eIF4GI, which serves as a scaffolding protein for the assembly of eIF4F, a protein complex composed of eIF4E (the mRNA cap-binding factor) and eIF4A (an ATP-dependent RNA helicase). Thus, via its association with the mRNA cap-binding protein eIF4E and with another translation initiation factor (eIF3) which is bound to the 40S ribosomal subunit, eIF4GI creates a physical link between the mRNA cap structure and the ribosome, thus facilitating cap-dependent translation initiation (25
). eIF4GI functions also in cap-independent, internal ribosome entry site (IRES)-mediated translation initiation. For instance, upon picornavirus infection, eIF4G is rapidly attacked by viral proteases. The resulting eIF4GI cleavage products serve to reprogram the cell's translational machinery, as the N-terminal cleavage product inhibits cap-dependent translation of host cell mRNAs by sequestering eIF4E while the C-terminal cleavage product stimulates IRES-mediated translation of viral mRNAs (23
). Similarly, apoptotic caspases cleave eIF4G into an N-terminal fragment that blocks cap-dependent translation and a C-terminal fragment that is utilized for IRES-mediated translation of mRNAs encoding proapoptotic proteins (22
The regulation of eIF4GI cleavage by viral proteases or apoptotic caspases has been extensively studied. Little is known, however, about the regulation of eIF4GI steady-state levels. Yet the eIF4GI amount that exists at a given moment results from the sum of the effects of de novo synthesis and ongoing degradation. Many cellular proteins are physiologically degraded by the proteasome. This has been shown to be true for eIF4GI, as the factor can be degraded by the proteasome in vitro
) and in living cells (6
). However, how eIF4GI targeting for or protection from destruction by the proteasome is regulated remains unknown.
There are two major routes to degradation by the proteasome. In the more conventional route, polyubiquitinated proteins are targeted to the 26S proteasome. Alternatively, a few proteins can be degraded by the 20S proteasome (and sometimes by the 26S proteasome) in a ubiquitin-independent manner (16
). Interestingly, it has been shown recently that a few of these proteins (1
) can be protected from degradation by the 20S proteasome by binding to the NAD(P)H quinone-oxydoreductase 1 (NQO1). It has been proposed that NQO1 may interact with the 20S proteasome and may consequently block access of target proteins to the 20S degradation core. Because eIF4GI can be degraded in vitro
by the 20S proteasome (5
) and since it appears that proteasomes can degrade eIF4GI in living cells independently of ubiquitination (6
), we asked whether NQO1 could protect eIF4GI from degradation by the proteasome.