Abstract
Laminins, the major basement membrane glycoproteins, are composed of three subunits. We identified a splice variant of the human laminin α4 subunit transcript containing 21 extra nucleotides. A heptapeptide sequence, MDCPTIS, was inserted close to the two cysteine residues possibly involved in the intersubunit disulfide bonds. Both the authentic α4 subunit (α4A) and the variant with the heptapeptide insertion (α4B) were readily secreted as laminin-8 trimers (α4Aβ1γ1 or α4Bβ1γ1) upon cotransfection with expression vectors for the β1 and γ1 subunits. The purified recombinant laminin-8 containing the α4B subunit was more potent in promoting cell spreading than that containing α4A, raising the possibility that the alternative splicing of the α4 subunit transcript regulates the cell-adhesive activity of laminin-8. Since both α4A and α4B transcripts were detected by RT-PCR in several human cell lines, these two isoforms of laminin-8 with differing cell-adhesive activities are present in the basement membranes of human tissues.
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