Molecular collaborations between serpins and trefoil factor promote endodermal cell growth and gastrointestinal differentiation in budding tunicates

We present evidence supporting novel collaborations between the serine protease inhibitor (serpin) and the trefoil factor during the budding stage of the tunicate Polyandrocarpa misakiensis. Using a maltose‐binding protein/P‐serpin fusion protein, two polypeptides of 40 kDa and 45 kDa were pulled down from Polyandrocarpa homogenates. Based on their partial amino acid sequence data, a single cDNA (928 bp) was cloned. It encodes a polypeptide that has five tandem repeats of a trefoil consensus motif. Thus, we termed the cDNA P‐trefoil. Both P‐trefoil and P‐serpin were expressed exclusively by coelomic cells during budding. P‐Trefoil was expressed mainly by coelomic cells throughout the asexual life cycle of Polyandrocarpa, while P‐Serpin was localized particularly in coelomic cells and in the extracellular matrix in developing buds. The native P‐Trefoil protein showed aminopeptidase activity. It induced cell growth in cultured Polyandrocarpa cells at a concentration of 8 µg/mL. P‐Serpin reinforced this activity of P‐Trefoil. Further, a mixture of P‐Trefoil and P‐Serpin exhibited the in vitro induction of a gut‐specific alkaline phosphatase. These results show for the first time that a serpin can interact with a trefoil factor to play a role in the cellular growth and differentiation of the gastric epithelium.