Cloning of a Zinc-binding Cysteine Proteinase Inhibitor in Citrus Vascular Tissue

Abstract

A partial cDNA (cvzbp-1) was cloned based on the N-terminal sequence of a citrus (Citrus L.) vascular Zn-binding protein (CVZBP) previously isolated from vascular tissue (Taylor et al., 2002). CVZBP has homology to the Kunitz soybean proteinase inhibitor (KSPI) family. Recombinant protein produced using the cDNA clone inhibited the cysteine proteinase, papain. Metal binding capacity has not been reported for any other member of this family. CVZBP was present in leaves, stems, and roots but not seeds of all citrus species examined. However, CVZBP was present in germinating seeds after the cotyledons had turned green. Within four hrs after wounding, CVZBP was undetectable in the wounded leaf and adjacent leaves. It has been suggested that many members of the KSPI family serve a function in defense. However, the expression of the CVZBP is in direct contrast with those of KSPI members that were implicated in defense response. Though systemically regulated during wounding, we suggest that CVZBP is not a defense protein but rather may function in vascular development.