Calpain 1-γ filamin interaction in muscle cells: A possible in situ regulation by PKC-α

Calpains are a family of calcium-dependant cysteine-proteases involved in cytoskeleton remodelling and muscle differentiation. In a recent study, we observed the presence of calpain 1 in the muscle contractile apparatus and specifically in the N1- and N2-lines.This calpain isoform was found to be involved in the degradation of muscle fibres via proteolysis of key proteins in Z-disk and costameric junctions.The goal of this study was to determine whether γ-filamin – a specific muscle isoform of the filamin family – is a calpain 1 substrate and to characterise this interaction. γ-Filamin is a major muscle architectural protein located in the Z-line and under the sarcolemmal membrane. This protein is a component of the chain binding the sarcolemma to the sarcomeric structure.In this study, we found that γ-filamin formed a stable complex in vitro and in cells with calpain 1 in the absence of calcium stimulation. We also located the binding domains in the C-terminus of γ-filamin with a cleavage site between serine 2626 and serine 2627 in the hinge 2 region. The catalytic (80 kDa) and regulatory (28 kDa) subunits of calpain 1 are both involved in high affinity binding at γ-filamin.Moreover, we showed that phosphorylation of the filamin C-terminus domain by PKCα protected γ-filamin against proteolysis by calpain 1 in COS cells. Stimulation of PKC activity in myotubes, prevented γ-filamin proteolysis by calpain and resulted in an increase in myotube adhesion.