Identification of a gene selectively expressed in the brain, which encodes a putative transmembrane protein and a soluble cytoplasmic isoform.

Subtractive cloning procedures led to the identification of a variety of transcripts expressed in mammalian brain. However, little is known about the encoded proteins and the regulation of gene expression. Here, we describe the isolation and characterisation of a single-copy gene (83.5) of 21.7 kb which is specifically expressed in porcine brain. In situ hybridisation and immunohistochemistry experiments showed a distinct pattern of gene expression in neuronal cell types in different parts of the brain. The gene contains two mini exons, confirming neural-specific expression. cDNA cloning experiments revealed two species of mRNA differing in their 5'-regions. These transcripts are generated by two distinct transcription start sites that are under the control of different potential promoter regions as shown by primer-extension experiments. The amino acid sequences of the deduced proteins predict that one mRNA species encodes a novel type-I transmembrane protein, whereas the other transcript encodes only a part of its cytoplasmic domain. In Western-blot experiments, we detected two proteins of the predicted size and cellular localisation in porcine brain. The precise function of these proteins remains to be determined. However, our findings suggest that they may be generated by alternative promoter usage, leading to the expression of a membrane protein and its truncated cytoplasmic isoform.