Rj. Kaufman et al., CHARACTERIZATION OF WILD-TYPE AND SER(53) MUTANT EUKARYOTIC INITIATION FACTOR-4E OVEREXPRESSION IN MAMMALIAN-CELLS, The Journal of biological chemistry, 268(16), 1993, pp. 11902-11909
Eukaryotic translation initiation factor 4E (eIF-4E) is one component
of the m7G-cap-binding protein complex eIF-4F and is required for cap-
dependent translation initiation. The phosphorylation state of eIF-4E
correlates with increased activity and a major phosphorylation site re
sides at serine 53. To further evaluate the role of eIF-4E phosphoryla
tion, eIF-4E wild-type and two Ser53 mutants, Ser53Ala and Ser53Asp, w
ere expressed at high level, representing almost 2% of the total cell
protein, by transient transfection of COS-1 monkey cells. (PO4)-P-32 m
etabolic labeling of transfected cells demonstrated both Ser53 mutants
were phosphorylated at an alternate serine residue. [S-35] Methionine
pulse-labeling demonstrated that the wild-type and both Ser53 mutants
were equally incorporated into the eIF-4F complex. The effect of wild
-type and Ser53 mutant overexpression on cap-dependent translation ini
tiation and internal translation initiation was monitored by cotransfe
ction with an expression vector encoding a dicistronic mRNA for which
the 5' cistron is translated in a cap-dependent manner, and the 3' cis
tron is translated by internal ribosome binding. Unexpectedly, overexp
ression of the wild-type or either mutant did not affect the efficienc
y of either cap-dependent or internal initiation. These results demons
trate that phosphorylation of eIF-4E at residue 53 is not required for
interaction with p220 and suggest that Ser'' phosphorylation may not
be required for cap-dependent translation initiation in this system.