A second eIF4E protein in Schizosaccharomyces pombe has distinct eIF4G-binding properties

The eukaryotic cap-binding proteins belonging to the! eIF4E family are gene rally involved in mediating the: recruitment of ribosomes to capped mRNA. W e described previously a cap-binding protein (now, called eIF4E1) in Schizo saccharomyces pombe that appears to have all of the usual structural and fu nctional attributes of an eIF4E. We have now characterised a new type of ca p-binding protein (eIF4E2) from this organism, which at the amino acid sequ ence level, is 52% identical and 59% similar to eIF4E1. eIF4E2 is not essen tial in S.pombe but has some: novel properties that may be related to a spe cial function in the cell. The ratio of eIF4E2:eIF4E1 in the: cell shifts i n favour of eIF4E2 at higher temperatures. Despite having all of the dorsal face amino acids that have so far been associated with eIF4G binding to eI F4E1, eIF4E2 binds the eIF4E-binding domain of S.pombe eIF4G >10(2)-times w eaker than eIF4E1 in vitro. The eIF4E2 cap-binding affinity is in the: typi cal micromolar range. The results suggest that eIF4E2 is not active on the main pathway, of translation initiation in fission yeast but might play a r ole in the adaptation strategy of this organism under specific growth condi tions. Moreover, they prov ide insight into the molecular characteristics r equired for tight binding to eIF4G.