Miraculin, a taste-modifying protein is secreted into intercellular spaces in plant cells

A taste-modifying protein, miraculin, is highly accumulated in ripe fruit of miracle fruit (Richadella dulcifica) and the content can reach up to 10% of the total soluble protein in these fruits. Although speculated for decades that miraculin is secreted into intercellular spaces in miracle fruit, no evidence exists of its cellular localization. To study the cellular localization of miraculin in plant cells, using miracle fruit and transgenic tomato that constitutively express miraculin, immunoelectron microscopy, imaging GFP fusion proteins, and immunological detection of secreted proteins in culture medium of transgenic tomato were carried out. Immunoelectron microscopy showed the specific accumulation of miraculin in the intercellular layers of both miracle fruit and transgenic tomato. Imaging GFP fusion protein demonstrated that the miraculin-GFP fusion protein was accumulated in the intercellular spaces of tomato epidermal cells. Immunological detection of secreted proteins in culture medium of transgenic tomato indicated that miraculin was secreted from the roots of transgenic tomato expressing miraculin. This study firstly showed the evidences of the intercellular localization of miraculin, and provided a new insight of biological roles of miraculin in plants.