NOVEL INHIBITORY-ACTION OF TUNICAMYCIN HOMOLOGS SUGGESTS A ROLE FOR DYNAMIC PROTEIN FATTY ACYLATION IN GROWTH CONE-MEDIATED NEURITE EXTENSION

In neuronal growth cones, the advancing tips of elongating axons and d endrites, specific protein substrates appear to undergo cycles of post translational modification by covalent attachment and removal of long- chain fatty acids. We show here that ongoing fatty acylation can be in hibited selectively by long-chain homologues of the antibiotic tunicam ycin, a known inhibitor of N-linked glycosylation. Tunicamycin directl y inhibits transfer of palmitate to protein in a cell-free system, ind icating that tunicamycin inhibition of protein palmitoylation reflects an action of the drug separate from its previously established effect s on glycosylation. Tunicamycin treatment of differentiated PC12 cells or dissociated rat sensory neurons, under conditions in which protein palmitoylation is inhibited, produces a prompt cessation of neurite e longation and induces a collapse of neuronal growth cones. These growt h cone responses are rapidly reversed by washout of the antibiotic, ev en in the absence of protein synthesis, or by addition of serum. Two a dditional lines of evidence suggest that the effects of tunicamycin on growth cones arise from its ability to inhibit protein long-chain acy lation, rather than its previously established effects on protein glyc osylation and synthesis. (a) The abilities of different tunicamycin ho mologues to induce growth cone collapse vary systematically with the l ength of the fatty acyl side-chain of tunicamycin, in a manner predict ed and observed for the inhibition of protein palmitoylation. Homologu es with fatty acyl moieties shorter than palmitic acid (16 hydrocarbon s), including potent inhibitors of glycosylation, are poor inhibitors of growth cone function. (b) The tunicamycin-induced impairment of gro wth cone function can be reversed by the addition of excess exogenous fatty acid, which reverses the inhibition of protein palmitoylation bu t has no effect on the inhibition of protein glycosylation. These resu lts suggest an important role for dynamic protein acylation in growth cone-mediated extension of neuronal processes.