Molecular cloning and functional expression of a water-soluble chlorophyll protein, a putative carrier of chlorophyll molecules in cauliflower.

A cDNA for a water-soluble chlorophyll (Chl) protein (WSCP) from cauliflower (Brassica oleracea L. var botrys) was cloned and sequenced. The cDNA contained an open reading frame encoding 19 residues for a signal peptide and 199 residues for the mature form of WSCP. The sequence showed extensive homology to drought-stress-related, 22-kDa proteins in some Brassicaceae plants. Functional WSCP was expressed in Escherichia coli as a fusion protein with a maltose-binding protein (MBP). When the recombinant MBP-WSCP was incubated with thylakoid membranes, the MBP-WSCP removed Chls from these membranes. During this process, the monomer of the apo-MBP-WSCP successfully bound Chls and was converted into tetrameric holo-MBP-WSCP. The reconstituted MBP-WSCP exhibited absorption and fluorescent spectra identical to those of the native WSCP purified from cauliflower leaves. The Chl a/b ratio in native WSCP indicates a high content of Chl a, which was mainly due to the higher affinity of MBP-WSCP for Chl a. WSCP is the first example of a hydrophilic protein that can transfer Chls from thylakoid hydrophobic proteins. Possible functions of WSCP are discussed.