Cultured human melanoma cells biosynthesize a novel form of laminin that lacks the M/sub r/ = 400 kDa subunit

A human melanoma cell line (A375) was found to produce an unusual type of laminin composed exclusively of M/sub r/ = 200 kDa B subunits and lacking the M/sub r/ = 400 kDa A subunit. Detergent lysates of A375 cells pulsed with [35S]methionine contained an immunoreactive form of laminin that migrated on nonreduced SDS-PAGE with an apparent M/sub r/ = 850 kDa. The 850 kDa laminin was clearly resolved from the typical M/sub r/ = 950 kDa laminin composed of A and B subunit types that is produced by a variety of cultured cells such as human choriocarcinoma (JAR) cells. Upon reduction of the intersubunit disulfide bonds of the A375 laminin, a pair of polypeptides co-migrated on SDS-PAGE with the M/sub r/ = 200 kDa B subunit doublet of JAR laminin. The A-deficient A375 laminin does not seem to be a result of the proteolysis of A subunit in cell lysates. No degradation of [35S]methionine-labeled JAR laminin was observed upon incubation with nonradioactive A375 cell lysate supplemented in the usual manner with ten protease inhibitors. Furthermore, A subunit did not appear transiently in A375 cells at any chase time (0-4 hr) following a 10-min biosynthetic pulse with [35S]methionine. The authors observations suggest that A375 cells biosynthesize principally the B subunits of laminin and assemble them to form a disulfide-linked macromolecule, possibly a B4 tetramer