Phosphorylation of maize eukaryotic translation initiation factor on Ser2 by catalytic subunit CK2

Abstract

Alignment of eukaryotic translation initiation factor 5A (eIF5A) sequences has shown, for plants, in contrast to most other eukaryotes, the presence of N-terminal serine residue (Ser2) which could be phosphorylated by CK2. Using point directed mutagenesis, we demonstrate here that in recombinant maize ZmeIF5Awt Ser2 is exclusively phosphorylated by catalytic subunit of CK2 (CK2α), whereas its mutated variant Ser2Ala is not phosphorylated. To shed light on the physiological significance of this Ser2 phosphorylation, transient expression of fluorescence-labeled proteins was performed in maize protoplast. Wild-type ZmeIF5A was distributed evenly between nucleus and cytoplasm, but the replacement of Ser2 by aspartic acid, which mimics the phosphorylated serine, influences its intracellular localization. We postulate that phosphorylation of Ser2 in maize eIF5A, and most probably in other plant cells, plays a role in specific regulation of nuclear export of eIF5A-bound mRNAs.

Identification of the Phosphorylated Residues in TveIF5A by Mass Spectrometry

“…In maize, S2 phosphorylation of eIF5A plays a unique role in regulating the intracellular localization of eIF5A and the nuclear export of mRNAs it binds to [18]. However, no phosphorylation was observed at S2 in TveIF5A; instead, phosphorylation was detected at S3 in the mature and intermediary TveIF5A isoforms.…”

“…Despite the absence in mammalian eIF5A [18], N-terminal S2 is highly conserved in plants and protozoan parasites examined, which could potentially be phosphorylated by CK2 [19]. In maize, S2 phosphorylation of eIF5A plays a unique role in regulating the intracellular localization of eIF5A and the nuclear export of mRNAs it binds to [18].…”

“…In higher eukaryotes, the positively charged hypusine residue in eIF5A facilitates its bind to exportin 4 (Exp4), enhancing the nuclear export of eIF5A [20]. On the other hand, the negative charge in the phosphorylated residues may prevent the interaction of eIF5A with its nuclear receptor, causing an accumulation of eIF5A in the nucleus [18]. Interestingly, human CK2 possesses a nuclear localization signal [21], which might phosphorylate and retain eIF5A in the nucleus [18].…”

“…On the other hand, the negative charge in the phosphorylated residues may prevent the interaction of eIF5A with its nuclear receptor, causing an accumulation of eIF5A in the nucleus [18]. Interestingly, human CK2 possesses a nuclear localization signal [21], which might phosphorylate and retain eIF5A in the nucleus [18]. A recent study showed that the phosphorylated proteins might have a crucial function in the morphological transformation of T. vaginalis from trophozoite to amoeboid or pseudocyst [22].…”

Quantitative phosphoproteomics reveals the role of the AMPK plant ortholog SnRK1 as a metabolic master regulator under energy deprivation

“…Moreover, depletion of eIF5A in yeast ( Saccharomyces cerevisiae ) rapidly affects the protein translation rate impairing it by 2–3-fold73. Further functions of phosphorylation of eIF5A might comprise transport processes of mRNA from the nucleus to the ribosome, thus initiating programmed cell death (PCD)7475.…”