DIDEMNIN BINDS TO THE PROTEIN PALMITOYL THIOESTERASE RESPONSIBLE FOR INFANTILE NEURONAL CEROID-LIPOFUSCINOSIS

The marine natural product didemnin B, currently in clinical trials as an antitumor agent, has several potent biological activities apparent ly mediated by distinct mechanisms. Our initial investigation of didem nin B resulted in the discovery of its GTP-dependent binding of the tr anslation elongation factor EF1 alpha. This finding is consistent with the protein synthesis inhibitory activity of didemnin B observed at i ntermediate concentrations. To begin to dissect the mechanisms involve d in the cytostatic and immunosuppressive activities of didemnin B, ob served at low concentrations, additional didemnin-binding proteins wer e sought. Here we report the purification of a 36-kDa glycosylated did emnin-binding protein from bovine brain lysate. Cloning of the human c DNA encoding this protein revealed a strong sequence similarity with p almitoyl protein thioesterase (PPT), an enzyme that removes palmitate from H-Ras and the G(alpha s) subunits of heterotrimeric GTP-binding p roteins in vitro. Mutations in PPT have recently been shown to be resp onsible for infantile neuronal ceroid lipofuscinosis, which is a sever e brain disorder characterized by progressive loss of brain function a nd early death.