Cloning and expression of carp cathepsin Z: Possible involvement in yolk metabolism

A cDNA encoding cathepsin Z (CTPZ) was cloned from a carp ovarian cDNA library. It is homologous to mammalian CTPZ. The amino acid residues important for protein folding and enzymatic activity of mammalian CTPZ are conserved in carp CTPZ. It is widely expressed in a variety of carp tissues as revealed by Western blot and reverse transcription-polymerase chain reaction. The CTPZ mRNA was transiently accumulated during oocyte maturation. In oocytes, CTPZ is localized in cortical granules and in the cytoplasm surrounding the yolk granules. After fertilization, CTPZ remained associated with the yolk granules while the cortical granular CTPZ was discharged to plasma membrane, perivitelline space, and fertilization envelope. Carp cathepsin Z has proteolytic activity toward vitellogenin that could be inhibited by inhibitors specific for the proteases of papain family. The potential roles of cathepsin Z in carp eggs are discussed.