CHARACTERIZATION OF 2 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. A lso present in the 5'-flanking region are two Alu repeats belonging to the Alu-Sa subfamily. When the determined genomic sequence was compar ed to a placental cDNA sequence [Oquendo et al., Cell 58 (1989) 95-101 ] and to a human erythroid leukemia (HEL) cell CD36 cDNA sequence (thi s report), we found that exons 2a and 2b do not occur within the same mRNA, suggesting that alternative splicing occurs within the 5'-untran slated region (UTR) of human CD36 pre-mRNA. These observations were co nfirmed by reverse transcriptase-coupled polymerase chain reaction (RT /PCR) assays using RNA from placental tissue, HEL cells and human plat elets. Exon 2b encodes two alternative translation initiation codons w hich may render exon 2b-containing CD36 mRNA untranslatable. To test t his 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 matu re protein observed in transfected COS-1 cells. Therefore, exon 2b its elf is insufficient to block translation of CD36 mRNA.