MYOSIN AND PARAMYOSIN OF CAENORHABDITIS-ELEGANS EMBRYOS ASSEMBLE INTONASCENT STRUCTURES DISTINCT FROM THICK FILAMENTS AND MULTI-FILAMENT ASSEMBLAGES

The organization of myosin heavy chains (mhc) A and B and paramyosin ( pm) which are the major proteins of thick filaments in adult wild-type Caenorhabditis elegans were studied during embryonic development. As a probe of myosin-paramyosin interaction, the unc-15 mutation e73 whic h produces a glu342lys charge change in pm and leads to the formation of large paracrystalline multi-filament assemblages was compared to wi ld type. These three proteins colocalized in wild-type embryos from 30 0 to 550 min of development after first cleavage at 20-degrees-C on th e basis of immunofluorescence microscopy using specific monoclonal ant ibodies. Linear structures which were diversely oriented around the mu scle cell peripheries appeared at 360 min and became progressively mor e aligned parallel to the embryonic long axis until distinct myofibril s were formed at 550 min. In the mutant, mhc A and pm were colocalized in the linear structures, but became progressively separated until th ey showed no spatial overlap at the myofibril stage. These results ind icate that the linear structures represent nascent assemblies containi ng myosin and pm in which the proteins interact differently than in wi ld-type thick filaments of myofibrils. In e73, these nascent structure s were distinct from the multifilament assemblages. The overlapping of actin and mhc A in the nascent linear structures suggests their possi ble structural and functional relationship to the ''stress fiber-like structures'' of cultured vertebrate muscle cells.