923 Mol. Biol. Evol. 15(7):923–925. 1998  1998 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038 Letter to the Editor A SINE that Acquired a Role in Signal Transduction During Evolution Mitsuru Shimamura, Masato Nikaido, Kazuhiko Ohshima, and Norihiro Okada Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan We have discovered a new SINE that appears to have acquired a novel role in signal transduction in the lineage of a group of artiodactyls. SINEs are a group of retroposons which have been amplified and integrated into genomes by retroposition (Okada and Ohshima 1995) The majority of SINEs are believed to be parasitic and to represent junk DNA in host organisms. However, in certain cases, SINEs appear to have gained novel functions and to have become involved in biological processes. More than 10 such examples have been re- corded, in which sequences of SINEs have become pro- moters that modify the expression of preexisting func- tional genes or provide a new polyadenylation signal (Britten 1996; Makalowski 1995). In other cases, SINEs have been integrated into the coding region and are translated into peptides, with resulting generation of pro- tein variability. In fact, more than 30 SINEs have been found in the coding regions of mRNAs (Makalowski 1995). However, no examples of such messages playing an explicit role in an important biological process have yet been reported (Makalowski 1995). We recently characterized the CHR-1 SINEs, which are distributed exclusively among whales, cows, and hippopotamuses (Shimamura et al. 1997). In a BLASTN search (Altschul et al. 1990), we found an example of a CHR-1 SINE that was inserted into the coding region of mRNA for the EP3 subtype of receptor for bovine prostaglandin (PG) E 2 . It appears that this message with an integrated SINE plays a key role in the signal trans- duction that is mediated by the bovine EP3 receptor for PGE 2 . Prostaglandin E 2 has a broad range of biological effects in diverse tissues that are mediated through its binding to specific receptors on plasma membranes. Four subtypes of receptors (EP1, EP2, EP3, and EP4) for PGE 2 have been recognized on the basis of their respective responses to agonists and antagonists (Cole- man et al. 1989). It has been proposed that the EP3 receptor mediates various very different effects of PGE 2 , such as inhibition of neurotransmitter release (Ohia and Jumblatt 1990), contraction of the uterus (Krall et al. 1984), inhibition of the reabsorption of sodium ions and water in kidney tubules (Nakao et al. 1989), and stim- ulation of catecholamine release from adrenal chromaf- fin cells (Tanaka et al. 1990). The EP3 receptor is a protein with seven trans- membrane domains. The bovine EP3 receptor exists as Key words: CHR-1, retroposon, bovine, prostaglandin E 2 , EP3, G protein. Address for correspondence and reprints: Nirhiro Okada, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Yokohama 226-8501, Japan. E-mail: nokada@bio.titech.ac.jp. at least four isoforms, which have 358 amino acids in common and different carboxy-terminal sequences (EP3A, EP3B, EP3C, and EP3D), which are produced by alternative splicing (Namba et al. 1993). We discov- ered that the exons that encode for the carboxy-terminal tails of bovine EP3B and EP3C are derived from part of the sequence complementary to the CHR-I SINE (fig. 1a). It has been demonstrated that the four different car- boxy-termini of specific isoforms are responsible for modulation of the coupling with different G proteins, such as Gi, Gs, and Go, that lead to the activation of different signaling pathways (Namba et al. 1993; Ne- gishi et al. 1993). Thus, EP3B and EP3C couple with Gs, which activates adenylate cyclase. EP3C has also been suggested to couple with Go, which is located in nervous tissue and excitable cells. By contrast, EP3A couples with Gi, which inhibits adenylate cyclase. To our knowledge, the bovine EP3 (isoforms B and C) re- ceptor is the first example of a SINE sequence, as part of an mRNA, that plays a key role in regulation as a translated protein. Of the 108 nucleotides of a CHR-1 SINE, 81 and 12 are translated into amino acids by using different frames in the carboxy-terminal tails of EP3B and EP3C, respectively (boxed nucleotides in fig. 1a). We eluci- dated a genome structure of this locus (fig. 1b, 1), sug- gesting a generation process of three differentially spliced mRNAs coding for EP3B, EP3C, and EP3A, re- spectively (figs. 1b, 2–4). In the present case, the coding sequence of the CHR-1 SINE is almost identical to the consensus sequence of the type II subfamily of the CHR-1 family (unpublished data), and almost no mu- tations have been introduced in this region. These ob- servations suggest the possibility that the putative amino acid sequence encoded by a unit of the CHR-1 SINE might be utilized in other receptors or subunits as a do- main for coupling to Gs or Go. According to this very attractive hypothesis, a retroposon can include a func- tional domain that can be shuffled among genomes, giv- ing rise to diverse physiological reactions. In any case, the present example demonstrates the apparent adaptive involvement of a SINE unit in the regulation of signal transduction, and it indicates that biological systems are more flexible than previously believed. Genes for the EP3 receptor have also been char- acterized in the human (Adam et al. 1994; Kunapuli et al. 1994; Yang et al. 1994; Schmid et al. 1995), mouse (Sugimoto et al. 1992), rat (Takeuchi et al. 1993; Neu- schafer-Rube et al. 1994), and rabbit (Breyer et al. 1994). In the human and the mouse, EP3 mainly couples with Gi and inhibits adenylate cyclase. Human EP3c and mouse EP3, which are homologous to bovine EP3D, form complexes not only with Gi, but also with Gs and Gp (Gq). Other subtypes of EP2 and EP4 couple with Gs and activate adenylate cyclase (Coleman et al. 1989; Downloaded from https://academic.oup.com/mbe/article/15/7/923/1074880 by guest on 19 June 2022