670C Poster - 10. Cell biology: Cytoskeleton, organelles and trafficking
Saturday April 09, 1:30 PM - 3:30 PM

Authors:
Kishen Chahwala; Cahir O'Kane

Affiliation: University of Cambridge

Keywords:
j. endoplasmic reticulum; a. neural degeneration

Axonal endoplasmic reticulum tubules are much narrower in axons than in most other locations. But what does this mean for lumenal protein diffusion? To test whether the small lumen diameter is limiting for protein diffusion, we compared the rate of diffusion of a GFP-tagged lumenal protein through wildtype larval Drosophila axonal ER tubules, with diffusion through mutant axonal ER tubules with a larger diameter, in a triple mutant lacking the ER-shaping proteins, Rtnl1, ReepA and ReepB. We performed FRAP experiments on axons to look at fluorescence recovery and thus the freedom of GFP to diffuse through the ER lumen.

We found that lumenal diffusion of GFP was significantly faster in the larger mutant ER tubules. The time to half recovery in wider axonal ER tubules (lacking Rtnl1, ReepA and ReepB) was approximately half as long as in wildtype.

The result implies that narrow axonal ER tubule diameter limits lumenal protein diffusion. This leaves open the question of why it might be important to limit lumenal ER diffusion along axons, for example, whether the diffusion of the much smaller (relative to GFP) Ca²⁺ ion is also constrained, and whether altered ER lumenal diffusion might be a factor in the pathology of diseases like Hereditary Spastic Paraplegia that are caused by mutations in ER-shaping proteins.