Molecular bases of bud development in ascidians

In the developing bud of the ascidian Polyandrocarpa misakiensis, many types of tissues are formed through transdifferentiation of the multipotent atrial epithelium. Several lines of evidence suggest that retinoic acid be involved in this process. We isolated cDNA clones encoding a retinoic acid receptor and retinoid X receptor from P. misakiensis. These homologs form a complex to bind to specific DNA sequences and function as retinoic acid-dependent transcriptional activators. Retinoic acid target genes were also identified. One of the cDNA clones encodes a modular protein containing a serine protease catalytic domain in the C-terminal region and several distinct protein-protein interaction domains in the N-terminal region. This protein is thus named tunicate retinoic acid-inducible modular protease (TRAMP). A recombinant TRAMP catalytic domain promoted proliferation of a cell line established from the atrial epithelium. These results suggest that TRAMP play a role in inducing dedifferentiation and/or stimulating the cell cycle.