Molecular characterization and expression of two ovarian lipoprotein receptors in the rainbow trout, Oncorhynchus mykiss

Partial cDNAs encoding two lipoprotein receptors were isolated and sequenced from the ovary in the rainbow trout (rt), Oncorhynchus mykiss. One of the cDNAs (rt-LPR) contained the 5 domains characteristic of receptors belonging to the low-density lipoprotein receptor superfamily. The second cDNA (rt-LP[OS]R) was similar to rt-LPR but lacked 105 base pairs encoding the O-linked sugar domain. The deduced amino acid sequences of the rt-LPR and rt-LP[OS]R had between 75% and 80% identity with very-low-density lipoprotein and vitellogenin receptors of other species. The rt-lipoprotein receptor mRNAs were approximately 3.5 kilobases in size. The rt-LPR was expressed in both the ovary and somatic tissues, whereas the rt-LP[OS]R was ovary-specific. Messenger RNA for the lipoprotein receptor(s) was expressed at high levels in both pre-vitellogenic (< 0.3 mm) and early vitellogenic (up to 1 mm) follicles. Using reverse transcription-polymerase chain reaction, expression of rt-LPR and rt-LP[OS]R mRNA was also detected in larger vitellogenic follicles (up to 2.5 mm in diameter) but not in follicles in late vitellogenesis or in ovulated eggs. The sequence, ovary specificity, and pattern of ovarian expression of the rt-LPR mRNA suggest that it encodes the receptor that mediates vitellogenin uptake into the ovary.