MS-desi, a desiccation-related protein in the floral nectar of the evergreen velvet bean (Mucuna sempervirens Hemsl): molecular identification and characterization.

Plant desiccation-related proteins (DRPs) were first identified as pcC13-62 from the resurrection plant Craterostigma plantagineum and it has been suggested they are involved in plant desiccation tolerance. We identified and characterized a plant DRP, which we called MS-desi, in the floral nectar of a subtropical bean species, Mucuna sempervirens (MS). MS-desi is a major nectar protein (nectarin) of the bean plant and expresses exclusively in the stylopodium, where the nectary is located. The full-length MS-desi gene encodes for a protein of 306 amino acids with a molecular mass of 33,248 Da, and possesses a ferritin-like domain and a signal peptide of 30 amino acids. Structural and phylogenetic analysis demonstrated MS-desi has high similarity to members of the plant DRPs, including pcC 13-62 protein. MS-desi has a similar hydropathy profile to that of pcC13-62 with a grand average of hydropathy index of 0.130 for MS-desi and 0.106 for pcC13-62 protein, which is very different from those of dehydrins and late embryogenesis abundant proteins. The protein's secondary structures, both predicted from the amino acid sequence and directly analysed by far UV circular dichroism, showed that MS-desi is mainly composed of alpha helices and is relatively temperature dependent. The structure change is reversible within a wide range of temperatures. Purified MS-desi and raw MS floral nectar showed dose-dependent citrate synthase inhibition activity, but insensitivity to lactate dehydrogenase, suggesting that, unlike dehydrins, it does not act as a chaperone. The overall results constitute, to our knowledge, the first study on a desiccation-related protein in plant floral nectar.