Localization and targeting of SCG10 to the trans-Golgi apparatus and growth cone vesicles

SCG10 is a membrane-associated, microtubule-destabilizing protein of neuron al growth cones. Using immunoelectron microscopy, we show that in the devel oping cortex of mice, SCG10 is specifically localized to the trans face Gol gi complex and apparently associated with vesicular structures in putative growth cones. Consistent with this, subcellular fractionation of rat forebr ain extracts demonstrates that the protein is enriched in the fractions con taining the Golgi apparatus and growth cone particles. In isolated growth c one particles, SCG10 was found to be particularly concentrated in the growt h cone vesicle fraction. To evaluate the molecular determinants of the spec ific targeting of SCG10 to growth cones, we have transfected PC12 cells and primary neurons in culture with mutant and fusion cDNA constructs. Deletio n of the amino-terminal domain or mutations within this domain that prevent ed palmitoylation at cysteines 22 and 24 abolished Golgi localization as we ll as growth cone targeting, suggesting that palmitoylation of the amino-te rminal domain is a necessary signal for Golgi sorting and possibly transpor t of SCG10 to growth cones. Fusion proteins consisting of the amino-termina l domain of SCG10 and the cytosolic proteins stathmin or glutathione-S-tran sferase colocalized with a Golgi marker, alpha-mannosidase II, and accumula ted in growth cones of both axons and dendrites. These results reveal a nov el axonal/dendritic growth cone targeting sequence that involves palmitoyla tion.