Proteolytic processing of human brain alpha spectrin (fodrin): identification of a hypersensitive site

The processing of brain spectrin (fodrin) by calcium-dependent proteases at the postsynaptic membrane has been postulated to be one of the central molecular mechanisms underlying long-term potentiation (LTP). The effect of such processing on the structure and function of brain spectrin, and on spectrin's ability to organize or otherwise regulate receptor function remains unclear. To address these issues, human and bovine brain spectrin were digested under mild conditions with several proteases, and the resulting cleavage fragments analyzed by 2-dimensional chymotryptic 125I peptide mapping. These studies identify an underlying protease-resistant domain structure reminiscent of, yet distinctly different from, human erythroid spectrin. More importantly, fodrin is unusual for the presence of a single, proteolytically hypersensitive site in the center of the alpha subunit, which is the favored site of action by many proteases, including the calcium-dependent neutral proteases. This proteolytically hypersensitive site is a unique feature of alpha nonerythroid spectrin since it is absent from human erythrocyte spectrin and appears to be the site at which the molecule is processed in vivo. In addition, on the basis of gel overlay techniques, it appears that the hypersensitive site is also the site at which calmodulin binds to the alpha-subunit in a calcium-dependent manner. These studies thus establish at the molecular level 2 calcium-dependent mechanisms by which brain spectrin function might be regulated and provide a conceptual and methodological framework for further investigation into the function of this important molecule.