Identification of a novel phosphorylation site of acyl-CoA binding protein (ACBP) in nodularin-induced apoptotic hepatocytes.

The liver specific protein phosphatase inhibiting toxin nodularin (from Nodularia spumigena) rapidly induces hepatocyte apoptosis. Incubation of freshly isolated hepatocytes with this toxin results in hyperphosphorylation of cellular proteins before any morphological signs of apoptosis appear. These phosphorylated proteins may play key roles in the early stage of apoptosis. Here, we identified one of the phosphoproteins to be acyl-CoA binding protein (ACBP), a highly conserved and ubiquitously expressed protein. Phosphorylation-site analysis by matrix-assisted laser desorption ionization time-of-flight MS/MS revealed that the observed phosphorylation is positioned on Ser1 in the N-terminal tryptic peptide Ac-SQADFDKAAE EVKRLK of the rat liver protein. Additionally, we observed a translocation of ACBP towards the cellular membrane in the apoptotic hepatocytes. Moreover, nodularin-induced apoptosis was highly dependent on calpain activation, an event that has previously been shown to be regulated by ACBP. Our findings introduce the possibility that reversible phosphorylation of ACBP regulates its ability to activate calpain in phosphatase inhibitor-induced apoptosis and controls the cellular accessibility of long-chain fatty acid-CoAs for cellular signaling.

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