Projectin is not released from calpain-digested myofibrils. Immunofluorescence microscopy images representing the staining pattern obtained on purified adult IFM myofibrils with or without calpain digestion. All left panels represent untreated myofibrils, and all right panels correspond to myofibrils treated with 72 μg of calpain/slide. Myofibrils were processed for immunofluorescence microscopy as described in Methods. Panels A and B: kettin (MAC155); panels C and D: myosin (3E8); panels E and F: F-actin (FITC-labeled phalloidin); panels G and H: projectin (Core 1P); panels I and J: projectin (3b11); and panels K and L: projectin (P5). White lines mark the position of the Z-bands. projectin is not released upon calpain digestion of the Z-bands. Scale bar in A is for panels A through D and I through L and represents 10 μm; scale bar in E is for panels E and F and represents 10 μm, scale bar in G is for panels G and H and represents 10 μm.