The Z-disc is a protein-rich structure critically important for myofibril development and integrity. Since a role of the Z-disc for signal integration and transduction was recently suggested, its precise phosphorylation landscape warranted in-depth analysis. We therefore established a site-resolved protein phosphorylation map of the Z-disc in skeletal myocytes and found that it is a phosphorylation hotspot in living cells, underscoring its functions in signalling and disease-related processes. In an exemplary fashion, we analysed the actin-binding multi-adaptor protein filamin C (FLNc), which is essential for Z-disc assembly and maintenance, and found that PKC phosphorylation at distinct serine residues in its hinge 2 region prevents its cleavage at an adjacent tyrosine residue by calpain 1. With this quantitative in vivo kinase assay, we show that the phosphorylation site S2625 in mouse FLNc is significantly down-regulated upon treatment of C2C12 myotubes with the PKCα inhibitor Gö6976.

Light, medium and heavy SILAC labelling of C2C12 cells was performed with 13C6 L-arginine and 12C6 L-lysine, 12C6 L-arginine and D4 L-lysine and 13C615N4 L-arginine and 13C615N2 L-lysine, respectively. Labelled myoblasts were seeded into six well plates and differentiation was induced by reduction of the dialyzed FCS content to 2% in the absence of sodium pyruvate. Cells were differentiated for at least 4d. Prior to signalling experiments, cells were starved 16 h overnight and sarcomere formation was improved by electrical pulse stimulation. Tryptic digests were seperated via high-pH reversed phase chromatography and 2 fractions per biological replicate containing the FLNc peptide GASYSSIPK were futher enriched for phosphorylated isoform by titanium dioxide enrichment. Treatment of the biological replicates was performed as follows: experiment 1: light - control; medium - PMA; heavy - Gö6976; experiment 2: light - Gö6976; medium - control; heavy - PMA, experiment 3: light - PMA; medium - Gö6976; heavy - control