IMMUNOCYTOCHEMICAL ELECTRON-MICROSCOPIC STUDY AND WESTERN-BLOT-ANALYSIS OF MYOSIN, PARAMYOSIN AND MINIPARAMYOSIN IN THE STRIATED-MUSCLE OF THE FRUIT-FLY DROSOPHILA-MELANOGASTER AND IN OBLIQUELY STRIATED AND SMOOTH MUSCLES OF THE EARTHWORM EISENIA-FOETIDA

Miniparamyosin is a paramyosin isoform (55-60 kDa) that has been isola ted in insects (Drosophila) and immunolocalized in several species of arthropods, molluscs, annelids and nematodes. In this study, the prese nce and distribution of this protein, in comparison with that of param yosin and myosin, has been examined in the striated muscle (tergal dep ressor of trochanter) of Drosophila melanogaster, and the obliquely st riated muscle (body wall) and the smooth muscle (outer layer of the ps eudoheart) of the earthworm Eisenia foetida by means of immunocytochem ical electron microscopic study and Western blot analysis miniparamyos in, paramyosin and myosin antibodies from Drosophila. In the striated muscle of D. melanogaster, the three proteins were immunolocalized alo ng the length of the thick filaments (A-bands). The distribution of im munogold particles along these filaments was uniform. The relative pro portions miniparamyosin/paramyosin/myosin (calculated by counting the number of immunogold particles) were: 1/10/68. In the obliquely striat ed muscle of E. foetidia, immunoreactions to the three proteins were a lso found in the thick filaments, and the relative proportions minipar amyosin/paramyosin/myosin were 1/2.4/6.9. However, whereas the distrib ution of both myosin and miniparamyosin along the thick filament lengt h was uniform, paramyosin immunolabelling was more abundant in the ext remes of thick filaments (the outer zones of A-bands in the obliquely striated muscle), where the thick filaments become thinner than in the centre (the central zone of A-bands), where these filaments are thick er. The relative proportions of paramyosin in the outer and of paramyo sin in the central zones of A-bands were 4/1. This irregular distribut ion of paramyosin along the thick filament length might be actual but it may also be explained by the fusiform shape of thick filaments in t he earthworm: assuming that paramyosin is covered by myosin, paramyosi n antigens would be more exposed in the tips than in the centre of thi ck filaments. If miniparamyosin is, in turn, covered by paramyosin, th e exposure of miniparamyosin antigens would be low even in the tips of thick filaments, and this might explain the scanty immunoreaction obs erved for this protein and the absence of a higher number of immunogol d particles in the extremes of thick filaments. The distribution of th e three proteins in the earthworm smooth muscle was as in the obliquel y striated muscle, although the proportions miniparamyosin/paramyosin/ myosin were 1/1.5/5.2; this is, immunoreactions to paramyosin and mini paramyosin were lower than in the obliquely striated muscle.