On the origin of mRNA encoding the truncated dopamine D₃-type receptor D(3nf) and detection of D(3nf)-like immunoreactivity in human brain

A truncated dopamine D₃-receptor-like mRNA, named D(3nf), predicts a protein that differs from the D₃-receptor only in the carboxyl terminus. However, such a protein has lost the predicted membrane topology typically found for G protein-coupled receptors. Results presented here show that D(3nf) mRNA arises from the D₃-encoded primary transcript via alternative splicing. This splicing, however, appears to involve cleavage of an unusual 3' splice site. Therefore, we tested the possibility that D(3nf) mRNA results from a splicing error. If this were the case, D(3nf) mRNA would be expected to be present in the cytoplasm only at very low amounts, and it would not be expected to be translated into protein. However, the relative abundance of cytoplasmic D₃/D(3nf) mRNA in human cortical tissues was found to be similar. Furthermore, we raised polyclonal antisera against the predicted carboxyl-terminal peptide sequence of D(3nf) that reacts specifically with a protein expressed in stably D(3nf) mRNA-expressing COS 7 cells. The use of this antiserum also revealed the presence of a ~68 kDa D(3nf)-like immunoreactive protein in human brain, suggesting that the atypically processed D(3nf) mRNA is translated.