An aspartic proteinase precursor, herein named BYC
(Boophilus Yolk pro-Cathepsin) was isolated from eggs of the hard
tick, Boophilus microplus. As judged by electrophoresis on sodium
dodecyl sulfate polyacrylamide slab gel (SDS–PAGE),
purified BYC presented 2 bands of 54 and 49 kDa, bearing the same
NH2-terminal amino acid sequence. By Western blot
analysis, BYC was also found in the haemolymph, indicating an extraovarian
site of synthesis. Several organs were incubated in culture medium with
[35S] methionine, and only the gut and fat body
showed synthesis of BYC polypeptides.
Protein sequencing of both the NH2-terminal and an internal
sequence obtained after cyanogen bromide (CNBr) cleavage
of BYC revealed homology with several aspartic proteinase precursors.
Incubation at pH 3·5 resulted in autoproteolysis of
BYC, which produced the mature form of the enzyme, that displayed
pepstatin-sensitive hydrolytic activity against
haemoglobin. Western blot analysis using anti-BYC monoclonal antibodies
showed proteolytic processing of BYC during
embryogenesis and suggested activation of the enzyme during development.
role of BYC in degradation of vitellin, the
major yolk protein of tick eggs, is discussed.