Structure of the mouse gene encoding CD4 and an unusual transcript in brain.

The T-cell differentiation antigen CD4 plays an important role in the function of T cells that recognize class II major histocompatibility complex proteins. Mouse CD4 (L3T4) has previously been shown to be evolutionarily related to immunoglobulin variable regions based on the predicted protein sequence from cDNA clones. The gene encoding L3T4 was found to be transcribed not only in a subset of T-lineage cells but also unexpectedly in brain, where a shorter transcript was found. In the present study the gene encoding L3T4 is shown to span 26 kilobases and to contain 10 exons. The structural organization is similar to that of other members of the immunoglobulin gene superfamily except for the striking presence of an intron in the middle of the sequence encoding the amino-terminal immunoglobulin-like homology unit. The structure of the shorter L3T4 transcript in mouse brain has been determined. This mRNA appears to be generated from a transcriptional start site within the coding sequence in exon VI. If translated, this transcript would encode a protein of 217 amino acids that lacks the usual L3T4 signal peptide and the amino-terminal 214 amino acids of the mature protein.