Amino acid sequence of a novel integrin beta 4 subunit and primary expression of the mRNA in epithelial cells.

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Abstract
Using the polymerase chain reaction, we have isolated cDNA clones that encode a new integrin beta subunit--beta 4. Its cDNA, which is 5676 bp in length, has one long coding sequence (5256 bp), a polyadenylation signal and a poly(A) tail. The deduced sequence of 1752 amino acids is unique among the integrin beta subunits. It contains a putative signal sequence as well as a transmembrane domain that divides the molecule into an extracellular domain at the N-terminal side and a cytoplasmic domain at the C-terminal side. The extracellular domain exhibits a 4-fold repeat of cysteine-rich motif similar to those of other integrin beta subunits. Certain features of the extracellular domain, however, are unique to the beta 4 subunit sequence. Of the 56 conserved cysteine residues found within the extracellular domain of other mature beta subunits, eight such residues are deleted from the beta 4 subunit sequence. The cytoplasmic domain is much larger (approximately 1000 amino acids) than those of other beta subunits (approximately 50 amino acids) and has no significant homology with them. A protein homology search revealed that the beta 4 subunit cytoplasmic domain has four repeating units that are homologous to the type III repetition exhibited by fibronectin. The beta 4 subunit mRNA was expressed primarily in epithelial cells. The restricted expression and the new structural features distinguish the integrin beta 4 subunit from other integrin beta subunits.