RA cannot affect the differentiated state of the atrial epithelium directly.  And RA-
treated mesenchyme cells can induce a secondary axis in the developing bud.
These results suggest that mesenchyme cells secrete transdifferentiation factor(s)
in response to RA.  We planned to isolate cDNAs for RA target genes of which
expression was activated by RA.
   Using a fluorescent differential display technique, we identified a few cDNA clones
(Figure 1).  One of them encoded a modular protein that contained serine protease
catalytic domain in the C-terminal region and several distinct protein-protein
interaction motifs in the N-terminal region.  This protein, named tunicate retinoic
acid-inducible modular protease (TRAMP), is a candidate transdifferentiation factor.
Another RA target gene was for a gonadotropin-releasing hormone receptor-like
seven pass G-protein coupled receptor. We are now analyzing functions of these
proteins.

Figure 1 (left): A result of the differential display analysis.  A PCR product indicated
by arrowhead were amplified only from a cDNA sample prepared from RA-treated
mesenchyme cells (+).
Figure 2 (right): A schematic drawing of the primary structure of a putative protein
product of a RA target gene named TRAMP.
 

References

Ohashi, M., Kawamura, K., Fujii, N., Yubisui, T. & Fujiwara, S. (1999)
   A retinoic acid-inducible modular protease in budding ascidians.
   Develop. Biol. 214:  38-45.
Fujiwara, S., Kamimura, M., Ohashi, M. & Kawamura, K. (2001) Molecular bases
   of bud development in ascidians. In The Biology of Ascidians. (ed. Sawada, H.,
   Lambert,C. & Yokosawa,H.) Elsevier Science Publisher, pp.300-304.
 

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