The CaaX proteases are closely related in the post-translational modification of many membrane-bound or secreted proteins and play a key role in the activation or stabilization of these molecules belonging to the CAAX family. In this study, a full-length cDNA putatively encoding a FACE-1/Ste24p CaaX protease (type I) of the Schistosoma japonicum was isolated. The cDNA, named SjSte24p, composed of 1646 bp and encoded 473 amino acids with predicted Mr/pI as 54.77 kDa/8.04. SjSte24p is a monoexonic gene constantly expressed in the parasite from cercariae to adult stages. It contained the characteristic of CaaX protease topology, including seven trans-membrane domains and a metallo-protease segment with a zinc-binding motif (HEXXH). SjSte24p shared a considerable degree of sequence identity with the type I CaaX proteases. A phylogenetic analysis showed that this protein family is tightly conserved from fungi to vertebrates. The expressed recombinant SjSte24p protein showed a proteolytic activity, which was inhibited by EDTA. Its activity was increased at low doses of the Zn2+ (0.001-0.01 mM); but was reversibly down-regulated at high doses (>0.1 mM). The native SjSte24p appeared to function in insoluble from. The protein was mainly localized in the tegument on the surface of adult worms. These results indicated that the SjSte24p is a practical zinc-dependent metalloprotease, which belongs to the FACE-1/Ste24p protease family.
Keywords: CaaX protease; FACE-1/Ste24p protease; Schistosoma japonicum; Zinc-dependent metalloprotease.
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