NcSUB1 (formerly known as NC-p65) is the first molecularly described proteolytic enzyme of the intracellular protozoan parasite Neospora caninum. This report describes the characterization of a rabbit anti-N54, which is an antiserum generated against an internal fragment of NcSUB1 (amino acids 649–783). In immunofluorescence studies rabbit anti-N54 labeled the apical end of the fixed parasite. By immuno-gold electron microscopy, the antibody bound primarily to the microneme organelles of the parasite. Analysis of secreted parasitic proteins indicated that a protein of molecular weight 65 kDa (reduced) or 55 kDa (nonreduced) was recognized by the antibody. The same secreted proteins were affinity purified with rabbit anti-N54–coupled resins and were shown to contain major proteolytic activity by zymography. Thus, rabbit anti-N54 is the first antibody developed for N. caninum that binds to the microneme proteins and recognizes a major secreted enzyme.

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