Fig. 3.

Axons contain SRP and ER components for co-translational secretion of locally synthesized proteins. A, SRP54 (red) of the signal recognition particle shows focal co-localization (arrow) with ribophorin II (green). Signal for peripherin is shown in blue. This image displays a three-dimensional average projection through the distal axon of the cultured DRG neurons (22 optical XY planes taken at 0.26 μm intervals in the z-axis). B, Colabeling cultures of DRG neurons for the α subunit of translocon-associated protein (TRAPα; red) and ribophorin II (green) is shown as a merged image DIC. Signals for ribophorin II and TRAPα extend into the distal axon and frequently overlap along the axon shaft and branch points (arrow). C, A three-dimensional average projection of axon from cultures stained for TRAPα, (green), protein disulfide isomerase (PDI; red), and neurofilament (blue) is shown (15 optical XY planes taken at 0.17 μm intervals). TRAPα and PDI focally overlap in the branch point and growth cone (arrows). D, Three-dimensional average projection of distal axon from DRG cultured neurons showing PDI (red), SERCA (green), and neurofilament immunoreactivity (blue). PDI and SERCA focally overlap in the branch point and distal regions of the axon (arrows). Neurofilament is shown in blue (10 optical XY planes taken at ~0.15 μm intervals; inset shows corresponding DIC image). E–F, Xenopus retinal ganglion cell growth cones stained for calreticulin (E) and ribophorin I (F). G–H, Mouse retinal ganglion cell growth cones stained for Sec61α (G) and PDI (H). I, Western blots on protein lysates from Rat DRG cultured neurons (SRP54 and PDI) and J, Western blots on Xenopus st38 brain/eye lysate (ribophorin I and calreticulin) and mouse E18 brain lysates (Sec61-alpha and PDI) demonstrate specificity of antibodies. [Scale bar: A, C=10 μm; B, D=5 μm; E, F=10 μm; G, H=5 μm].