2 ALTERNATIVE FORMS OF CDNA-ENCODING CD34

By expression cloning using FACS, we have isolated cDNA clones encodin g human CD34 from a megakaryoblastoid cell line. The predicted amino a cid sequence of CD34 revealed the type I transmembrane protein consist ed of a leader peptide (31 residues), an extracellular domain (258 res idues), a transmembrane domain (23 residues) and a cytoplasmic domain of 73 residues. In addition, a second form of cDNA that has 194 bp ins ertion in the cytoplasmic region was isolated. The analysis of genomic DNA showed that this sequence is inserted between the predicted trans membrane and cytoplasmic exons due to an alternative usage of an imper fect 5' splice acceptor site in the 5' flanking region of the cytoplas mic exon. The insertion brings a stop codon so that the protein encode d by this type of mRNA has only 16 residues in the cytoplasmic domain. This truncated form of CD34 molecule can be expressed on the cell sur face, and its expression seems to change in association with cell diff erentiation. A search of the National Biomedical Research Foundation P rotein Sequence Database (NBRF) with the Predicted amino acid sequence of CD34 did not reveal homology to any known protein. Thus, the CD34 molecule represents a novel type of cell surface molecule that may hav e a role in early differentiation process of hematopoietic stem cells.