THE IDENTIFICATION OF A NEW ALTERNATIVE EXON WITH HIGHLY RESTRICTED TISSUE EXPRESSION IN TRANSCRIPTS ENCODING THE MOUSE PGP-1 (CD44) HOMINGRECEPTOR - COMPARISON OF ALL 10 VARIABLE EXONS BETWEEN MOUSE, HUMAN, AND RAT

The human CD44 cell surface glycoprotein family which has been implica ted in lymphocyte homing, tumor metastasis, and extracellular matrix a ttachment consists of a large number of related isoforms that derive f rom the differential splicing of a single CD44 gene transcript. Recent mapping of the CD44 locus in man indicated the presence of at least n ine alternative exons within the region of the gene encoding the varia ble membrane proximal extracellular domain. Here we report the identif ication of a 10th alternative exon (termed V1) in the mouse, human, an d rat CD44 genes. We demonstrate tissue-specific patterns of expressio n for transcripts containing exons V1-V10 in the mouse and a highly re stricted usage of exon V1 in transcripts from mouse gastric tissue. Cl ose sequence homology between exons V1-V10 from mouse rat and human po ints to a specific functional role rather than a purely structural rol e for the membrane proximal extracellular domain of the CD44 molecule.