Molecular characterization and expression of vitellogenin receptor from white perch (Morone americana)

A full-length (4021 base pair [bp]) cDNA encoding a polypeptide (844 amino acids) with a predicted mass of 93 kDa and other characteristic structural features of a vertebrate vitellogenin receptor (VgR) was isolated from a white perch (Morone americana) ovarian cDNA library. Northern blotting performed using a specific digoxygenin-labeled VgR cDNA probe revealed a distinct approximately 4.1 kilobase (kb) hybridization signal in an mRNA preparation obtained from previtellogenic perch ovaries. The deduced amino acid sequence of the perch VgR was 89% and 82% identical, respectively, to that of the tilapia and rainbow trout. Because it possessed an eight-repeat ligand-binding domain (LR8) but lacked an O-linked sugar domain (-), the perch VgR was identified as a non-O-linked form of VgR (LR8-). Unlike the case in other vertebrates investigated, including tilapia and trout, no species of mRNA encoding an O-linked form of VgR (LR8+) could be detected when perch ovarian or liver mRNA reverse transcripts or cDNA libraries were screened by PCR using primer sets flanking the putative O-linked sugar domain. These novel findings call into question the assumptions that an LR8+ splice variant of the VgR always is dominantly present in somatic tissues and exists at lower levels in ovarian tissues to sequester lipoproteins distinct from Vg. A SYBR-green-based real-time reverse transcription-polymerase chain reaction assay was developed and used to quantitatively measure VgR expression in gonadal and somatic tissues, for the first time in any vertebrate. The main site of perch VgR mRNA expression was the ovary and the highest level of VgR mRNA expression was in ovaries whose largest follicles contained previtellogenic oocytes. Expression of VgR mRNA decreased with oocyte growth during vitellogenesis and was very limited in ovulated eggs. These quantitative results verify the concept that growing oocytes must extensively recycle LR8- forms of the VgR.