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Germ cells give rise to the next generation and contain ribonucleoprotein particles, germ granules. In these granules, Piwi protein Aubergine has been shown to interact with Tudor protein in Drosophila. Tudor protein has 11 Tudor domains and it has been unclear to what extent all these domains are involved in the interaction with Aubergine. Here we present direct biochemical evidence that Tudor-Aubergine interaction surface is composed of different Tudor domains including those that have not been previously implicated in Aubergine recognition. Furthermore, we show that specific single Tudor domains determine localization of Tudor complex to different sites in ovarian germ cells. Our data suggest that multiple Tudor domains of germline proteins from various species are redundantly used for interaction with the same protein partner during germline development.
Copyright © 2010 Elsevier Inc. All rights reserved.
The early cell cycles of Drosophila embryogenesis involve rapid oscillations between S phase and mitosis. These unique S-M cycles are driven by maternal stockpiles of components necessary for DNA replication and mitosis. Three genes, pan gu (png), plutonium (plu), and giant nuclei (gnu) are required to control the cell cycle specifically at the onset of Drosophila development by inhibiting DNA replication and promoting mitosis. PNG is a protein kinase that is in a complex with PLU. We employed a sensitized png mutant phenotype to screen for genes that when reduced in dosage would dominantly suppress or enhance png. We screened deficiencies covering over 50% of the autosomes and identified both enhancers and suppressors. Mutations in eIF-5A and PP1 87B dominantly suppress png. Cyclin B was shown to be a key PNG target. Mutations in cyclin B dominantly enhance png, whereas png is suppressed by cyclin B overexpression. Suppression occurs via restoration of Cyclin B protein levels that are decreased in png mutants. The plu and gnu phenotypes are also suppressed by cyclin B overexpression. These studies demonstrate that a crucial function of PNG in controlling the cell cycle is to permit the accumulation of adequate levels of Cyclin B protein.
Only serine phosphorylation of eukaryotic initiation factor-4E (eIF-4E) has been previously reported in intact cells. We found that treatment of HepG2 cells with okadaic acid resulted in as much as 20% of eukaryotic initiation factor (eIF)-4E phosphorylation occurring on threonine residues and that tryptic phosphopeptide maps showed several previously unrecognized phosphopeptides. Analysis of p220 from control and okadaic acid-treated cells demonstrated serine and threonine phosphorylation under both conditions. However, a unique pattern of phosphopeptides in okadaic acid-treated cells was observed. The most notable finding was that hyperphosphorylation of eIF-4E and p220 increased binding of p220 but not eIF-4E to the m7GTP cap structure. We suggest that phosphorylation of eIF-4E is more complicated than previously recognized and that hyperphosphorylation of eIF-4E and p220 recruits more p220 into the protein complex that associates with mRNA caps. A better understanding of these protein-protein and protein-mRNA interactions may aid the design of anti-sense directed chemistries that disrupt such interactions for a specific target mRNA (Baker, B.F., Miraglia, L., and Hagedorn, C. H. (1992) J. Biol. Chem. 267, 11495-11499).
The 25 kDa mRNA cap binding protein can be purified in a partially phosphorylated state and the extent of its phosphorylation appears to be regulated during heat shock and mitosis in mammalian cells. We demonstrated that a nonabundant serine protein kinase activity exists in rabbit reticulocytes that phosphorylates the 25 kDa cap binding protein in both the free (eIF-4E) and complexed (eIF-4F) state. This kinase was not inhibited by the cAMP-dependent protein kinase inhibitory peptide IAAGRTGRRNAIHDILVAA, did not phosphorylate S6 ribosomal protein, did not phosphorylate p220 of eIF-4F as protein kinase C does and no other substrates for this kinase were apparent in reticulocyte ribosomal salt wash. The molecular identity of this kinase, the specific site(s) of eIF-4E that it phosphorylates and its in vivo regulatory role remain to be studied.
The 25-kDa mRNA cap-binding protein (eIF-4E) exists in both phosphorylated and dephosphorylated forms in eukaryotic cells. Phosphorylated eIF-4E appears to be preferentially associated with 48 S initiation complexes and with the 220-kDa subunit of eIF-4F. In addition, dephosphorylation of eIF-4E has been observed during heat shock and mitosis which are accompanied by decreased protein synthesis. However, the control of eIF-4E phosphorylation and its regulatory role remain poorly understood. Using eIF-4E as a substrate we have identified and purified from rabbit reticulocytes a protein kinase that phosphorylates eIF-4E in vitro. This enzyme phosphorylated eIF-4E on both serine and threonine residues with an apparent Km of 3.7 microM. The molecular mass of the enzyme and specificity for substrates other than eIF-4E suggested that this enzyme was a species of casein kinase I. This was confirmed by comparing the phosphopeptide map of the purified reticulocyte enzyme with that of rabbit skeletal muscle casein kinase I and by comparing phosphopeptide maps of eIF-4E phosphorylated in vitro by each enzyme. We conclude that casein kinase I phosphorylates eIF-4E in vitro and suggest that eIF-4E may be phosphorylated by casein kinase I in intact cells under some physiologic conditions.
Several lines of evidence indicate that phosphorylation of the 25 kDa mRNA cap binding protein (eIF-4E) stimulates the efficiency of translational initiation. While the protein kinases which catalyze this reaction in intact cells have not been completely identified, evidence suggests that protein kinase C phosphorylates serine residues of eIF-4E in intact cells. In this study we demonstrate that protein kinase C also phosphorylates threonine residues of recombinant human eIF-4E in vitro. Phosphorylation of threonine and serine was observed over a range of eIF-4E and salt concentrations. However, relatively low levels of phosphorylation were seen even under optimal conditions. Similar results were observed with native eIF-4E purified from human erythrocytes. These findings demonstrate that protein kinase C can phosphorylate both serine and threonine residues of eIF-4E in vitro, but suggest that protein kinase C may not be the primary enzyme that phosphorylates eIF-4E in vivo.
Bacterial lipopolysaccharide (LPS) produces rapid changes in macrophage protein synthesis and function. Phosphorylation of the 25 kDa mRNA cap-binding protein (eIF-4E) in model systems regulates the efficiency of protein synthesis. We report that both LPS and tumor necrosis factor-alpha (TNF-alpha) stimulate phosphorylation of eIF-4E and the p220 component of eIF-4F in bone marrow-derived macrophages. Moreover, anti-TNF-alpha antibodies inhibit LPS-stimulated phosphorylation of eIF-4E and p220 by 43% (+/- 6%) and 50% (+/- 5%), respectively. Our results indicate that LPS stimulates eIF-4F phosphorylation by a TNF-alpha-dependent mechanism, and suggest that phosphorylation of eIF-4F might play a role in the post-transcriptional regulation of gene expression in macrophages exposed to LPS.