Immunocytochemical Observations on Muscle Cell Proteins in the Antarctic Mussel Shrimp Acetabulastoma sp. (Crustacea, Ostracoda)

Abstract

In the antarctic ostracode Acetabulastoma sp., a singular type of muscle cell 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 structural proteins in this muscle by electron microscopical immunocytochem- istry. Troponin, caldesmon, and calponin are three proteins suitable to identify muscle cell types that do not show the classical ultrastructural patterns expected of striated or smooth muscles. Reaction to troponin T was positive (the protein was located along the thin filaments), but no immunoreaction was observed to caldesmon and calponin. Thus, the muscle clearly belongs to the striated class. The contractile proteins myosin, paramyosin, and miniparamyosin were lo- cated along the thick filaments. Paramyosin and miniparamyosin are known only from inver- tebrate thick filaments and have no known vertebrate homolog. A nebulin-like protein was found and, as in vertebrates, may be involved in regulating sarcomere length. Instead of the giant protein titin, known from vertebrate striated muscle, minititin, a protein of the same family but lower molecular weight, was developed. The presence and distribution of proteins such as myosin, troponin, and nebulin as well as the absence of caldesmon and calponin suggest that despite its small size and parasitic life style, Acetabulastoma sp. is an active invertebrate quite unlike those in which the contractile proteins were found to be discontinuously distributed or concentrated at the tips of the thick filaments.