Hence, we suggest that longer mRNAs are affected less than shorter mRNAs by the elimination of eIF4G because the eIF4F cap interaction is inherently less stable for longer transcripts and, hence, less efficacious in promoting 43S recruitment when eIF4G is present. The fact that depleting eIF4G diminishes, selelck kinase inhibitor but does not eliminate the correlation Inhibitors,Modulators,Libraries between TE and ORF length indicates that reduced eIF4G PABP interaction is not the only factor limiting the translation of mRNAs with longer ORFs, and limited processivity of elongating ribosomes or less efficient ter mination have been suggested as other possibilities. We showed previously that depletion of eIF4G did not lower the amounts of native 48S complexes containing the RPL41A or MFA2 mRNAs, both very short tran scripts, which is ostensibly at odds with the idea that eIF4G has an important function in 43S attachment to mRNA.

Examining the results we obtained for these mRNAs in the LP dataset reveals that they both exhibit mean TE4G values 90% of their TEWT values. Thus, even if we assume that these two mRNAs require eIF4G only at the step of 43S attachment to achieve their maximum translation rates, it would have been Inhibitors,Modulators,Libraries very difficult to detect a 10% decrease in the levels of their free 48S complexes with the techniques employed in the previous study. It remains to be determined what features in mRNA, besides a short 5UTR and short ORF length, are responsible for the more pronounced requirement Inhibitors,Modulators,Libraries for eIF4G displayed by the small fraction of yeast mRNAs identified here.

Considering that eIF4G is essential in yeast, and also noting its role as a protein bridge linking the eIF4E mRNA PABP mRNP to components of the 43S Inhibitors,Modulators,Libraries complex, it is surprising that a significant amount of translation still proceeds in the absence of this factor. Based on our microarray data, it appears that eIF4G is dispensable for the translation of most, if not all mRNAs in vivo, indicating that it is rate enhancing rather than essential in budding yeast. This stands in Inhibitors,Modulators,Libraries contrast to the critical requirement for the eIF3 com plex, which is required for nearly all translation in yeast, and is crucial for attachment of native 43S complexes to mRNAs that can assemble 48S PICs in cells depleted of eIF4G. Of course, we can not exclude the possibility that a compensatory initia tion pathway comes into play during the 8 h of incubation in the non permissive conditions used to thoroughly deplete eIF4G.

It is also impossible to elimi nate the possibility that a very small fraction of the WT amount of eIF4G, below the detection selleck Cilengitide limit of our Wes tern analysis, is sufficient to catalyze the residual protein synthesis that occurs in the depleted cells. This seems unlikely, however, because the eIF4G level in WT cells is already lower than those of nearly all other initiation factors.