Axonal transport of clathrin-associated proteins

Clathrin, the main constituent of coated vesicles, is anterogradely transported exclusively in the slow component b (SCb) of axonal transport. However, it has not been shown whether the 30-36-kDa clathrin-associated proteins (CAPs), which may regulate assembly of clathrin into coated vesicles, are transported along with clathrin in SCb. Clarification of this point has implications for the functional state of anterogradely transported clathrin. To investigate CAPs transport, retinal ganglion cells of the guinea pig were labeled with 35S-methionine and the optic nerves harvested at 6 h, 4 days, and 30 days to collect radiolabeled proteins from each major rate component of axonal fast component (FC), slow component a (SCa), and SCb. The radiolabeled rate component proteins were analyzed by using two-dimensional polyacrylamide gel electrophoresis and fluorography. The results showed that CAPs, like clathrin, were transported exclusively with the proteins of SCb. In addition, a comparison of radiolabeled CAPs isolated from axons with whole-brain CAPs failed to demonstrate an appreciable difference in molecular weight or isoelectric point between the two, suggesting that CAPs did not undergo a major post translational modification upon passage into the synaptic terminal. It appears that the distinctive microenvironment within the synaptic terminal is likely to contribute to the ability of clathrin and CAPs to interact with membranes.