Figure 10.

Myosin undergoes proteolysis in IFM of fln⁰adults. (A) Immunoblot analysis of skinned IFM fiber proteins separated on a 10% SDS-PAGE. The blot was incubated with an anti–MHC mAb. Lanes b are IFM proteins from adult flies aged 2–4 d; lanes a are IFM proteins from newly eclosed adults, <5 h old. Full-length MHC is detected in all samples. However, note the presence of the ∼150-kD peptide (arrow) in the aged Mhc¹³ and fln⁰, but not in the OR (wild-type) and hdp² samples. The absence of site-specific myosin proteolysis in hdp² is significant since this TnI mutation leads to severe fiber degeneration and sarcomere breakdown. (B) In vitro proteolysis of IFM skinned fibers with endoproteinase Arg-C. Skinned fibers from <15-min-old adults were incubated without (−) or with (+) endoproteinase Arg-C and partial digestion products separated on a 10% SDS-PAGE. An immunoblot incubated with anti–MHC mAb 3E8 is shown. A 150-kD peptide appears in Mhc¹³- and fln⁰-treated fibers, but not in OR- or hdp²-treated fibers. Since flightin is absent in Mhc¹³and fln⁰, it may protect an MHC protease-sensitive region in vivo.