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Calcium‐activated Proteolysis of Intermediate Filaments a

Intermediate filaments (IF) are a major component of the cytoskeleton in different cell types. While the functions of IF remain unclear, it is speculated that some IF, especially neurofilaments (NF), provide structural stability to the cytoskeleton, enabling markedly asymmetrical cell forms to be established and maintained. Large neurons, for example, contain axonal processes whose dimensions are several thousand fold greater than those of the parent neuronal cell body. It seems reasonable to suspect that the synthesis, assembly, and transport of N F down the axon play an important role in determining dimensions and configuration of the neurons, especially its axonal extension. Most studies on NF metabolism have focused on their synthesis and axonal transport. Much less information is available on the fate of these proteins although it is clear that the enormous quantities of NF proteins transported down the axon must be balanced by a comparable process of degradation and turnover. There is, in fact, considerable evidence suggesting that calcium-activated protease is a major factor regulating NF metabolism. This paper will review data on calcium-mediated breakdown of N F in situ, the isolation and characterization of calcium-activated proteases that degrade N F proteins, and the nature of the reaction whereby the enzyme degrades isolated N F proteins and N F in situ.