Hypoxia enhances phosphorylation of eukaryotic initiation factor 4A in maize root tips.
We have identified two isoforms of initiation factor 4A (eIF-4A) in maize root tips, with distinct isoelectric points and similar molecular mass (approximately 50 kDa). Both isoforms of maize eIF-4A cross-react with antibodies raised against wheat germ eIF-4A, and one of the maize proteins (higher pI isoform) comigrates with purified wheat germ eIF-4A on two-dimensional gels. The two maize eIF-4As were indistinguishable by comparative peptide fingerprint analysis, which also showed a very strong similarity between eIF-4A in maize roots and wheat germ. Maize eIF-4As copurify with eIF-4F and eIF-(iso)4F on a 7-methyl-GTP-Sepharose affinity column, indicating that they are part of the 5'-cap-binding complex. Two-dimensional gel electrophoresis and immunoblotting of proteins from 32P-labeled maize root tips revealed that the lower pI isoform of eIF-4A is phosphorylated. Two-dimensional phosphopeptide maps of trypsin-digested eIF-4A contained one principal phosphorylated fragment; phosphoamino acid analysis indicated phosphorylation of threonine. In oxygenated maize root tips, the ratio of phosphorylated to nonphosphorylated eIF-4A is approximately 0.2. This ratio increases to approximately 1 within 20 min following the onset of hypoxia, due to interconversion between the two maize eIF-4A isoforms. The hypoxia-induced phosphorylation of eIF-4A is discussed with respect to metabolic responses, and the translational control of gene expression, in hypoxic plant tissues.
Webster
C
C
Department of Biochemistry, University of California, Riverside 92521.
Gaut
R L
RL
Browning
K S
KS
Ravel
J M
JM
Roberts
J K
JK
D015180:Electrophoresis, Gel, Two-Dimensional; D039601:Eukaryotic Initiation Factor-4A; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism*; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010766:Phosphorylation; D013912:Threonine / Q000378:metabolism; D003313:Zea mays / Q000235:genetics*
D010448:Peptide Initiation Factors; D013912:Threonine; D039601:Eukaryotic Initiation Factor-4A; D010100:Oxygen