Nervous picket fences

A neuron is a dual-function device—it does both input and output—but it has only one continuous plasma membrane. Protein transport helps to define the two different compartments, but without a barrier membrane, proteins will eventually intermix. Now, Chieko Nakada, Akihiro Kusumi (ERATO and Nagoya University, Nagoya, Japan), and colleagues confirm that neurons do have a membrane diffusion barrier, and they propose a mechanism by which it is constructed.

A diffusion barrier at the axonal initial segment (IS; the axonal area nearest the cell body) has been proposed before. But there have always been caveats: the introduced dye might have had too far to travel, or the latex bead used during testing might have cross-linked and thus immobilized its target.

The Japanese team used three single-molecule techniques to demonstrate that diffusion of a...