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INSERM UMR 641, Institut Jean Roche, Université de la Méditerranée, Faculté de Médecine Secteur-Nord, 13916 Marseille cedex 20, France

Address correspondence to Bénédicte Dargent, INSERM UMR 641, Institut Jean Roche, Université de la Méditerranée, Faculté de Médecine Secteur-Nord, Boulevard P. Dramard, 13916 Marseille cedex 20, France. Tel.: 33-491 698 859. Fax: 33-491 090 506. email: dargent.b{at}jean-roche.univ-mrs.fr

The axonal initial segment is a unique subdomain of the neuron that maintains cellular polarization and contributes to electrogenesis. To obtain new insights into the mechanisms that determine protein segregation in this subdomain, we analyzed the trafficking of a reporter protein containing the cytoplasmic II–III linker sequence involved in sodium channel targeting and clustering (Garrido, J.J., P. Giraud, E. Carlier, F. Fernandes, A. Moussif, M.P. Fache, D. Debanne, and B. Dargent. 2003. Science. 300:2091–2094). Here, we show that this reporter protein is preferentially inserted in the somatodendritic domain and is trapped at the axonal initial segment by tethering to the cytoskeleton, before its insertion in the axonal tips. The nontethered population in dendrites, soma, and the distal part of axons is subsequently eliminated by endocytosis. We provide evidence for the involvement of two independent determinants in the II–III linker of sodium channels. These findings indicate that endocytotic elimination and domain-selective tethering constitute a potential mechanism of protein segregation at the axonal initial segment of hippocampal neurons.

Key Words: axonal initial segment; ankyrin G tethering; sorting; neuronal polarity; sodium channels

J. J. Garrido's present address is Centro de Biología Molecular "Severo Ochoa," Universidad Autónoma de Madrid, 28049 Cantoblanco, Spain.

Abbreviations used in this paper: AIS, axonal initial segment; [AIS], AIS concentration; BFA, brefeldin A; CAM, cell adhesion molecules; DA, distal axon; MAP2, microtubule associated protein 2; PB, phosphate buffer; SD, somatodendritic.

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