Brassica napus Drought-Induced 22-kD Protein (BnD22) Acts Simultaneously as a Cysteine Protease Inhibitor and Chlorophyll-Binding Protein.

Class II water-soluble chlorophyll proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins that bind with chlorophyll (Chl) and its derivatives. The physiological function of WSCPs is still unclear, but it is assumed to be involved in stress responses, which is likely related to their Chl-binding and protease inhibition (PI) activities. Yet, the dual function and simultaneous functionality of WSCPs must still be better understood. Here, the biochemical functions of Brassica napus drought-induced 22-kDa protein (BnD22), a major WSCP expressed in B. napus leaves, were investigated using recombinant hexahistidine-tagged protein. We showed that BnD22 inhibited cysteine proteases, such as papain, but not serine proteases. BnD22 was able to bind with Chla or Chlb to form tetrameric complexes. Unexpectedly, BnD22-Chl tetramer displays higher inhibition toward cysteine proteases, indicating (i) simultaneous Chl-binding and PI activities and (ii) Chl-dependent activation of PI activity of BnD22. Moreover, the photostability of BnD22-Chl tetramer was reduced upon binding with the protease. Using three-dimensional structural modeling and molecular docking, we revealed that Chl binding favors interaction between BnD22 and proteases. Despite its Chl-binding ability, the BnD22 was not detected in chloroplasts but rather in the endoplasmic reticulum and vacuole. In addition, the C-terminal extension peptide of BnD22, which cleaved off post-translationally in vivo, was not implicated in subcellular localization. Instead, it drastically promoted the expression, solubility and stability of the recombinant protein.