In bacteria, nascent proteins bear the pretranslationally generated N-terminal (Nt) formyl-methionine (fMet) residue. Nt-fMet of bacterial proteins is a degradation signal, termed fMet/N-degron. By contrast, proteins synthesized by cytosolic ribosomes of eukaryotes were presumed to bear unformylated Nt-Met. Here we found that the yeast formyltransferase Fmt1, although imported into mitochondria, could also produce Nt-formylated proteins in the cytosol. Nt-formylated proteins were strongly up-regulated in stationary phase or upon starvation for specific amino acids. This up-regulation strictly required the Gcn2 kinase, which phosphorylates Fmt1 and mediates its retention in the cytosol. We also found that the Nt-fMet residues of Nt-formylated proteins act as fMet/N-degrons and identified the Psh1 ubiquitin ligase as the recognition component of the eukaryotic fMet/N-end rule pathway, which destroys Nt-formylated proteins.

Although Nα-terminal acetylation (Nt-acetylation) is a pervasive protein modification in eukaryotes, its general functions in a majority of proteins are poorly understood. In 2010, it was discovered that Nt-acetylation creates a specific protein degradation signal that is targeted by a new class of the N-end rule proteolytic system, called the Ac/N-end rule pathway. Here, we review recent advances in our understanding of the mechanism and biological functions of the Ac/N-end rule pathway, and its crosstalk with the Arg/N-end rule pathway (the classical N-end rule pathway).

Aim: Protein L-isoaspartyl O-methyltransferase (PIMT) regulates cell adhesion in various cancer cell lines through activation of integrin αv and the PI3K pathway. The epithelial mesenchymal transition (EMT) enables epithelial cells to acquire the characteristics of mesenchymal cells, and to allow them to migrate for metastasis. Here, we examined the relationship between PIMT and EMT with attached or detached MDA-MB 231 cells. Methods: Human breast cancer cell line MDA-MB-231 cells were maintained in a suspension on poly-HEMA in the presence or absence of PIMT siRNA or ERK inhibitor PD98059. The mRNAs and proteins were analyzed using RT-PCR and immunoblotting, respectively. Results: During cellular incubation under detached conditions, PIMT, integrin αv and EMT proteins, such as Snail, Slug and matrix metalloproteinase 2 (MMP-2), were significantly increased in correlation with the phosphorylation of ERK1/2. The ERK inhibitor PD98059 (25 µmol/L) strongly suppressed the expression of the proteins and PIMT. Interestingly, PIMT siRNA blocked the phosphorylation of ERK and the expression of the EMT proteins. Additionally, PIMT and ERK phosphorylation were both co-activated by treatment with TGF-β (10 ng/mL) and TNF-α (10 ng/mL). Conclusion: A tight cross-regulation exists between ERK and PIMT in regards to their activation and expression during the EMT. Because the role of PIMT in the EMT and metastatic processes remains unclear, in this study, we explored the involvement of PIMT in the regulation of the detachment and attachment of the anoikis-resistant cell line MDA-MB-231, an aggressive breast cancer cell line with a highly invasive, migrative, and metastatic characters [10] , by culturing the cells in poly-HEMA (2-hydroxyethylmethacrylate)-coated dishes and introducing siRNA specific for PIMT. Materials and methods MaterialsLiCl and poly-HEMA were purchased from Sigma-Aldrich (St Louis, MO, USA). Dulbecco's modified Eagle's medium (DMEM), penicillin/streptomycin solution (10 000 unit/mL and 10 mg/mL, respectively), fetal bovine serum (FBS) and Dulbecco's phosphate-buffered saline (DPBS) were purchased from Gibco BRL (Gaithersburg, MD, USA). MDA-MB 231 cells were obtained from American Type Culture Collection (Manassas, VA, USA). Moloney murine leukemia virus (M-MLV) reverse transcriptase, polymerase chain reaction (PCR) premix, and Sapphire Super Taq were purchased from Rexgene Biotech Co, Ltd (Ochang, Korea). All primers used for PCR were purchased from Bioneer (Taejeon, Korea). A mixture of Stealth TM /siRNA duplex oligoribonucleotides against PIMT and Lipofectamine TM RNAiMAX was purchased from Invitrogen (Carlsbad, CA, USA). Rabbit anti-PIMT antiserum was produced against recombinant PIMT proteins of porcine brain as previously described (Koh and Hong, unpublished data). Antibodies against MAPK, MMP-2, MMP-9, N-cadherin, integrin αv, phospho-GSK3 (Tyr 279/216 ), phospho-ERK1/2 (Thr 202 /Tyr 204 ), phospho-MEK1 (Ser 218/222 )/MEK2 (Ser 222/226 ), phospho-Akt1/PkBα (Ser 473 ) and phospho-p90RSK (Ser 380 ) were...

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We aimed to study the role of protein L-isoaspartyl methyltransferase (PIMT) in neuronal differentiation using basic fibroblast growth factor (bFGF)-induced neuronal differentiation, characterized by cell-body shrinkage, long neurite outgrowth, and expression of neuronal differentiation markers light and medium neurofilaments (NF). The bFGF-mediated neuronal differentiation of PC12 cells was induced through activation of mitogen-activated protein kinase (MAPK) signaling molecules [MAPK kinase 1/2 (MEK1/2), extracellular signal-regulated kinase 1/2 (ERK1/2), and p90RSK], and phosphatidylinositide 3-kinase (PI3K)/Akt signaling molecules PI3Kp110β, PI3Kp110γ, Akt, and mTOR. Inhibitors (adenosine dialdehyde and S-adenosylhomocysteine) of protein methylation suppressed bFGF-mediated neuronal differentiation of PC12 cells. PIMT-eficiency caused by PIMT-specific siRNA inhibited neuronal differentiation of PC12 cells by suppressing phosphorylation of MEK1/2 and ERK1/2 in the MAPK signaling pathway and Akt and mTOR in the PI3K/Akt signaling pathway. Therefore, these results suggested that PIMT was critical for bFGF-mediated neuronal differentiation of PC12 cells and regulated the MAPK and Akt signaling pathways. [BMB Reports 2016; 49(8): 437-442]

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