Immunocytochemical observations on muscle cell proteins in the antarctic mussel shrimp Acetabulastoma sp (Crustacea, Ostracoda)

In the antarctic ostracode Acetabulastoma sp., a singular type of muscle ce ll has tentatively been classified as transversely striated with continuous Z-line, interrupted by tubules of sarcoplasmic reticulum. We investigated the presence and distribution of different regulatory, contractile, and str uctural proteins in this muscle by electron microscopical immunocytochemist ry. Troponin, caldesmon, and calponin are three proteins suitable to identi fy muscle cell types that do not show the classical ultrastructural pattern s expected of striated or smooth muscles. Reaction to troponin T was positi ve (the protein was located along the thin filaments), but no immunoreactio n was observed to caldesmon and calponin. Thus, the muscle clearly belongs to the striated class. The contractile proteins myosin, paramyosin, and min iparamyosin were located along the thick filaments. Paramyosin and minipara myosin are known only from invertebrate thick filaments and have no known v ertebrate homolog. A nebulin-like protein was found and, as in vertebrates, may be involved in regulating sarcomere length. Instead of the giant prote in titin, known from vertebrate striated muscle, minititin, a protein of th e same family but lower molecular weight, was developed. The presence and d istribution of proteins such as myosin, troponin, and nebulin as well as th e absence of caldesmon and calponin suggest that despite its small size and parasitic life style, Acetabulastoma sp. is an active invertebrate quite u nlike those in which the contractile proteins were found to be discontinuou sly distributed or concentrated at the tips of the thick filaments.