MOLECULAR-CLONING OF THE HUMAN ALPHA-6 INTEGRIN SUBUNIT - ALTERNATIVE SPLICING OF ALPHA-6 MESSENGER-RNA AND CHROMOSOMAL LOCALIZATION OF THE ALPHA-6-GENE AND BETA-4-GENE

We have isolated cDNAs encoding the alpha-6 subunit from a lambda-gt11 expression library from human keratinocytes by combined screening with a rabbit polyclonal anti-alpha-6 antibody and the polymerase chain reaction. The alpha-6 subunit encoded by this cDNA consists of 1050 amino acids with a 991-amino-acid extracellular, a 23-amino-acid transmembrane and a 36-amino-acid cytoplasmic domain. The extracellular domain contains three putative divalent cation-binding sites and nine potential N-linked glycosylation sites. From a cDNA library from normal human mammary gland cells two different cDNAs for alpha-6 were isolated, one of which is identical to the above cDNA. The two alpha-6 subunits, called alpha-6A and alpha-6B, encoded by the two cDNAs each have a unique cytoplasmic domain, that of alpha-6B being 18 amino acids longer than that of alpha-6A. Different carcinoma cell lines contain transcripts for both alpha-6 subunits. K562 leukemic cells have little alpha-6A or alpha-6B mRNAs. The overall level of expression varies in the carcinoma cell lines, but reflects alpha-6 cell surface expression. In A375 melanoma cells, however, cell surface expression of alpha-6 was low in spite of a high level of mRNA. This suggests that other mechanisms may be involved in regulating the expression of alpha-6 on the surface of these cells. The mRNA for both alpha-6 subunits is around 6 kb. The alpha-6 subunits are similar to other or subunits (26-31% identity with cleaved a subunits) of the integrin family but they are more similar to the alpha-3 subunit (40% identity). This high degree of similarity may be the basis for their functional resemblance since both alpha-3 and alpha-6 subunits, when associated with beta-1, function as laminin receptors and bind to the long arm of laminin. The genes for alpha-6 and beta-4, the alternative beta-subunit with which alpha-6 combines on certain epithelial cells, were mapped to chromosome 2 and 17q11-qter, respectively.