MARP1 increases passive tension in mouse myofibrils. (A) Typical force and sarcomere length traces of a stretched myofibril of m. tibialis cranialis of a MKO mouse in the presence and absence of 831 nM GST-mMARP1. Inset shows a typical myofibril mounted in the experimental setup. (B) Peak and plateau passive force at various concentrations of GST-mMARP1. Each bar mean ± SEM is the average of 12 myofibrils. (C) STED microscopy images of myofibrils exposed to GST-only (top) or GST-mMARP1 (bottom), followed by α-N2A and α-MARP1 antibody staining. Note that N2A and MARP1 colocalize. T test relative to (−) GST-mMARP1. ***, P < 0.001; ****, P < 0.0001.