RESTRICTION OF 480 270-KD ANKYRIN(G) TO AXON PROXIMAL SEGMENTS REQUIRES MULTIPLE ANKYRIN(G)-SPECIFIC DOMAINS/

Ankyrin(G) (-/-) neurons fail to concentrate voltage-sensitive sodium channels and neurofascin at their axon proximal segments, suggesting t hat ankyrin(G) is a key component of a structural pathway involved in assembly of specialized membrane domains at axon proximal segments and possibly nodes of Ranvier (Zhou, D., S. Lambert, D.L. Malen, S. Carpe nter, L. Boland, and V. Bennett, manuscript submitted for publication) . This paper addresses the mechanism for restriction of 270-kD ankyrin (G) to axon proximal segments by evaluation of localization of GFP-tag ged ankyrin(G) constructs transfected into cultured dorsal root gangli on neurons, as well as measurements of fluorescence recovery after pho tobleaching of neurofascin-GFP-tagged ankyrin(G) complexes in nonneuro nal cells. A conclusion is that multiple ankyrin(G)-specific domains, in addition to the conserved membrane-binding domain, contribute to re striction of ankyrin(G) to the axonal plasma membrane in dorsal root g anglion neurons. The ankyrin(G)-specific spectrin-binding and tail dom ains are capable of binding directly to sites on the plasma membrane o f neuronal cell bodies and axon proximal segments, and presumably have yet to be identified docking sites. The serine-rich domain, which is present only in 480- and 270-kD ankyrin(G) polypeptides, contributes t o restriction of ankyrin(G) to axon proximal segments as well as limit ing lateral diffusion of ankyrin(G)-neurofascin complexes. The membran e-binding, spectrin-binding, and tail domains of ankyrin(G) also contr ibute to limiting the lateral mobility of ankyrin(G)-neurofascin compl exes. Ankyrin(G) thus functions as an integrated mechanism involving c ooperation among multiple domains heretofore regarded as modular units . This complex behavior explains ability of ankyrin(B) and ankyrin(G) to sort to distinct sites in neurons and the fact that these ankyrins do not compensate for each other in ankyrin gene knockouts in mice.