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General Comment |
Cachot J, et al reported the molecular cloning of flounder Xp18, a newly identified highly conserved protein
mainly expressed in the ovary.
Screening of a flounder ovary cDNA library with a rainbow trout p53 probe led to the isolation of a
p53-unrelated cDNA encoding an unknown 161 amino acid protein. In view of its apparent
molecular weight and yet unknown function, the deduced protein was named Xp18. Corresponding
orthologous cDNAs or expressed sequence tags have been identified in several species including
human, rodents, bovine, chicken and zebrafish and a related cDNA has also been isolated in the fruit
fly. Deduced amino acid sequences appeared to be extremely well conserved throughout vertebrate
evolution. Structure predictions suggested that Xp18 may correspond to an integral protein
comprising four transmembrane domains. The charged C-termini of all known vertebrate Xp18-like
proteins displayed a characteristic KXKXX motif which is considered as an endoplasmic reticulum
targeting sequence. Gene expression, as shown by Northern blot and quantitative reverse
transcription-polymerase chain reaction analysis, was significantly higher in the ovary and to a
lesser extent in the brain. Xp18 transcripts were also detected by in situ hybridization in most of the
circumventricular regions of the brain of adult flounders. The gene encoding the human protein is
located on chromosome Xq22.1, a genome region involved in numerous genetic diseases including
premature ovarian failure.
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