THE DIFFUSION OF MOLECULES IN AXONAL PLASMA-MEMBRANES - THE SITES OF INSERTION OF NEW MEMBRANE MOLECULES AND THEIR DISTRIBUTION ALONG THE AXON SURFACE

The neuronal cell surface consists of two domains, the somatodendritic and axonal plasma membranes. Each domain serves different functions, and has a different complement of membrane molecules. Since membrane m olecules are able to diffuse in the plane of the plasma membrane lipid bilayer, with diffusion coefficients ranging from 10(-8) cm(2) s(-1) for lipids to 10(-10) cm(2) s(-1) for proteins, mechanisms must exist to prevent the intermixing of membrane molecules from each domain by d iffusion. Presented here is a theoretical analysis of the distribution of axonal molecules in both growing and non-growing axone based on tw o models for the insertion of these molecules into the axonal membrane , namely insertion exclusively at the distal end of the axon, or inser tion with equal probability all along the axon. In all cases, assuming that the molecules have a finite half-life in the axonal membrane, co mpositional differences between the axonal and somatodendritic membran es can be obtained that are similar to those observed in other polariz ed cells, even in the absence of a physical barrier to prevent the int ermixing of membrane molecules. Moreover, our analyses demonstrate tha t the diffusion of membrane molecules in the plane of the axonal membr ane lipid bilayer is a sufficiently slow process to preclude the possi bility that membrane molecules are inserted into axonal membranes at a site remote from their final location, and then move to their final l ocations by diffusion. Thus, in long axons, for membrane molecules tha t are localized all along the length of the axon, mechanisms must exis t for their insertion in the axonal membrane at sites all along the ax on, and not just at the distal end. (C) 1998 Academic Press.