DIFFERENT TEMPORAL PATTERNS OF EXPRESSION RESULT IN THE SAME TYPE, AMOUNT, AND DISTRIBUTION OF FILAMIN (ABP) IN CARDIAC AND SKELETAL MYOFIBRILS

The morphogenesis of functional myofibrils in chick skeletal and cardi ac muscle occurs in greatly different time spans, in about 7 and 2 day s, respectively. In chick skeletal myogenic cells, one isoform of the 250 kD actin-binding protein (ABP) filamin is associated with stress f iber-like structures of myoblasts and early myotubes, then disappears for approximately 4 days, whereupon a second filamin isoform reappears at the Z-disc periphery. We sought to determine if cardiac myogenesis involves this sequence of appearance, disappearance, and reappearance of a new filamin isoform in a compressed time scale. It was known tha t in mature heart, filamin is localized at the Z-disc periphery as in mature (fast) skeletal muscle, and is also associated with intercalate d discs. We find that myocardial filamin has an apparent molecular wei ght similar to that of adult skeletal muscle filamin and lower than th at of smooth muscle filamin, and that both skeletal and cardiac muscle contain roughly 200 filamin monomers per sarcomere. Two-dimensional p eptide mapping shows that myocardial filamin is very similar to skelet al muscle filamin. Myocardial, slow skeletal, and fast skeletal muscle filamins are all phosphorylated, as previously shown for filamin of n on-striated muscle. Using immunofluorescence, we found that filamin co uld not be detected in the developing heart until the 14-somite stage, when functional myofibrils exist and the heart has been beating for 3 to 4 hours. We conclude that in cardiac and skeletal myogenesis, diff erent sequences of filamin gene expression result in myofibrils with s imilar filamin distributions and isoforms. (C) 1994 Wiley-Liss, Inc.