Characterization of two alternatively spliced 5'-untranslated exons of the human CD36 gene in different cell types.

We determined the nucleotide sequence of a 2.6-kb BamHI-EcoRI fragment from the 5'-end of the human gene encoding the cell adhesion receptor, CD36. This region contains the first coding exon, exon 3, as well as two non-coding exons, exons 2a and 2b, from the 5'-flanking region. Also present in the 5'-flanking region are two Alu repeats belonging to the Alu-Sa subfamily. When the determined genomic sequence was compared to a placental cDNA sequence [Oquendo et al., Cell 58 (1989) 95-101] and to a human erythroid leukemia (HEL) cell CD36 cDNA sequence (this report), we found that exons 2a and 2b do not occur within the same mRNA, suggesting that alternative splicing occurs within the 5'-untranslated region (UTR) of human CD36 pre-mRNA. These observations were confirmed by reverse transcriptase-coupled polymerase chain reaction (RT/PCR) assays using RNA from placental tissue, HEL cells and human platelets. Exon 2b encodes two alternative translation initiation codons which may render exon 2b-containing CD36 mRNA untranslatable. To test this hypothesis, we transfected COS-1 cells with an exon 2b-containing CD36 cDNA construct. Using anti-CD36 polyclonal antibody, we were able to detect an immunoreactive protein, consistent in size with the mature protein observed in transfected COS-1 cells. Therefore, exon 2b itself is insufficient to block translation of CD36 mRNA.