Cloning and characterization of mr-s, a novel SAM domain protein, predominantly expressed in retinal photoreceptor cells.
BACKGROUND
Sterile alpha motif (SAM) domains are approximately 70 residues long and have been reported as common protein-protein interaction modules. This domain is found in a large number of proteins, including Polycomb group (PcG) proteins and ETS family transcription factors. In this work, we report the cloning and functional characterization of a novel SAM domain-containing protein, which is predominantly expressed in retinal photoreceptors and the pineal gland and is designated mouse mr-s (major retinal SAM domain protein).


RESULTS
mr-s is evolutionarily conserved from zebrafish through human, organisms through which the mechanism of photoreceptor development is also highly conserved. Phylogenetic analysis suggests that the SAM domain of mr-s is most closely related to a mouse polyhomeotic (ph) ortholog, Mph1/Rae28, which is known as an epigenetic molecule involved in chromatin modifications. These findings provide the possibility that mr-s may play a critical role by regulating gene expression in photoreceptor development. mr-s is preferentially expressed in the photoreceptors at postnatal day 3-6 (P3-6), when photoreceptors undergo terminal differentiation, and in the adult pineal gland. Transcription of mr-s is directly regulated by the cone-rod homeodomain protein Crx. Immunoprecipitation assay showed that the mr-s protein self-associates mainly through the SAM domain-containing region as well as ph. The mr-s protein localizes mainly in the nucleus, when mr-s is overexpressed in HEK293T cells. Moreover, in the luciferase assays, we found that mr-s protein fused to GAL4 DNA-binding domain functions as a transcriptional repressor. We revealed that the repression activity of mr-s is not due to a homophilic interaction through its SAM domain but to the C-terminal region.


CONCLUSIONS
We identified a novel gene, mr-s, which is predominantly expressed in retinal photoreceptors and pineal gland. Based on its expression pattern and biochemical analysis, we predict that mr-s may function as a transcriptional repressor in photoreceptor cells and in pinealocytes of the pineal gland.
Inoue
Tatsuya
T
Osaka Bioscience Institute, 6-2-4 Furuedai, Suita, Osaka 565-0874, Japan. inouet@obi.or.jp
Terada
Koji
K
Furukawa
Akiko
A
Koike
Chieko
C
Tamaki
Yasuhiro
Y
Araie
Makoto
M
Furukawa
Takahisa
T
D000818:Animals; D002460:Cell Line; D003001:Cloning, Molecular; D005136:Eye Proteins / Q000235:genetics* / Q000378:metabolism; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D051379:Mice; D051857:Otx Transcription Factors / Q000235:genetics* / Q000378:metabolism; D010786:Photoreceptor Cells / Q000502:physiology*; D010870:Pineal Gland / Q000502:physiology; D063146:Polycomb-Group Proteins; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics* / Q000378:metabolism; D012160:Retina / Q000502:physiology*; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism
D005136:Eye Proteins; D018398:Homeodomain Proteins; D051857:Otx Transcription Factors; D063146:Polycomb-Group Proteins; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; C109288:cone rod homeobox protein; C510151:mr-s SAM domain protein, mouse