Characterization of different sizes of rat luteinizing hormone/chorionic gonadotropin receptor messenger ribonucleic acids.

The LH/CG receptor is uniquely expressed in the gonads of both sexes at specific stages of development. In ovaries, its expression marks particular steps of the ovulation cycle. An enigmatic aspect of expression of the LH/CG receptor is the dramatically diverse transcript sizes [from 7 to < 1 kilobase (kb)] and development-dependent expression of different sizes of mRNAs. It has been thought that mRNAs larger than 2.1 kb encode full-length receptors, whereas those smaller than 2.1 kb encode truncated receptor, because the full-length coding sequence is 2.1 kb. As a first step in elucidation of these diverse mRNAs and corresponding proteins, we have produced a series of cDNA clones and determined their DNA sequences and deduced the amino acid sequences of the resulting proteins. Our data demonstrate that variant mRNAs are produced by alternate splicing and polyadenylation, and they encode significantly shorter truncated receptor peptides. Surprisingly, many of these variant mRNAs are larger than 2.1 kb, and some are 4.2 kb. Some of them are polyadenylated in introns 3, 4, and 10. These alternate mRNAs were successfully expressed in 293 cells to produce receptor peptides 81, 116, and 294 amino acids in length compared to the wild-type receptor, which consists of 674 amino acids. Although these receptor peptides are not secreted, they are capable of binding the hormone, indicating the presence of a hormone contact site(s) in the short peptide fragments, particularly the N-terminal 81-amino acid segment. The data presented here will be helpful for understanding the functions of different sizes of mRNAs and also be valuable in studies designed to investigate whether individual cells express a specific message or multiple messages and how different classes of LH/CG receptor mRNAs are selectively expressed dependent on differentiation and development of the gonads.