The life cycle of Acanthamoeba is divided into a growth-division phase and two distinctive processes of cellular differentiation, termed encystment and excystment. Polyacrylamide gel electrophoresis revealed that a specific protein of 21 kDa in molecular weight occurs in the cyst, but not in the trophozoite stages of A. castellanii Neff strain. This cyst-specific protein, designated as CSP21, was purified from guanidine-HCl extract of cyst wall and anti-CSP21 antibody was produced. Immunoblotting of proteins extracted from a variety of species of Acanthamoeba genus suggested that the antibody is specific for group II amoebae, therefore, providing a useful tool for Acanthamoebae taxonomy. A cDNA clone for A. castellanii CSP21 was isolated by immunoscreening of a cDNA expression library constructed from mRNA of amoebae at encysting stage. The deduced primary structure indicated that CSP21 is a hydrophilic protein showing no significant homology with peptides thus far published. RNA blot analysis showed that the expression of CSP21 mRNA was restricted within early stages of encystment, suggesting that the biosynthesis of CSP21 is regulated at mRNA level.
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