CHARACTERIZATION OF CDNA CLONES ENCODING A MAJOR MICRONEME ANTIGEN OFSARCOCYSTIS-MURIS (APICOMPLEXA) CYST MEROZOITES

Two monoclonal antibodies directed against a microneme antigen of Sarc ocystis muris cyst merozoites (16/17 kDa band doublet) were used to is olate cDNA clones from a lambda ZAP expression library. Restriction an alysis revealed that the inserts were highly similar, with sizes rangi ng between 1.8 and 2.3 kb. In addition, a full-length cDNA insert of 2 .6 kb was obtained by hybridization screening. On Northern blots, a si ngle mRNA species of 2.7 kb was detected by a cDNA-derived probe. Sout hern blot hybridization suggests that the gene is present as a single copy. The nucleotide sequence of the full-length clone contains a sing le reading frame with a coding capacity of 26.5 kDa. The hypothetical polypeptide consists of a putative N-terminal signal peptide followed by a hydrophilic domain of unknown function, and the mature protein se quence. After purifying the 16/17 kDa antigen from cyst merozoites, a partial N-terminal amino acid sequence was obtained. Thus, the identit y of the cDNA sequence was confirmed. The deduced sequence of the matu re protein is predominantly hydrophilic and rich in cysteine (8.7%). D atabase searching suggested weak homologies of the hypothetical polype ptide to plasma kallikrein, tenascin and blood coagulation factors.