The acidic basic repeat
antigen (ABRA) of Plasmodium falciparum has been localised on the merozoite surface and in the parasitophorous vacuole. It is one of the
antigens enriched in the clusters of merozoites formed with growth inhibitory immune serum and possesses
chymotrypsin-like activity.
Chymostatin, an inhibitor of
chymotrypsin, inhibits
malaria invasion as well as autoproteolysis of ABRA. Based on these characteristics of ABRA, it seems important for invasion and should be investigated as a target for
vaccine and
drug design. For the functional characterisation of this
protein, the full-length mature ABRA
protein and its fragments with/without the putative
protease active site were cloned, expressed and purified from Escherichia coli. The polyclonal serum raised against recombinant ABRA fragment recognised a parasite
protein with a mobility of 101 kDa in an immunoblot assay and showed immunofluorescence activity with a schizont-rich preparation of P. falciparum. Using a partially purified fragment containing the putative active site and fluorogenic and
chromogenic substrates, we established that the
protease activity of ABRA resides in the N-terminal portion of the
protein and the highly charged C-terminal part of the
protein is not required for this activity. The
protease activity of ABRA was inhibited with
serine protease inhibitors like
chymostatin and phenyl methyl
sulfonyl fluoride (PMSF) whereas
leupeptin was not able to inhibit this
enzyme activity. These results clearly indicated that ABRA is a
protease with
chymotrypsin-like specificity. This is the first report describing the expression and characterisation of recombinant ABRA
protein.