Sequencing of laminin B chain cDNAs reveals C-terminal regions of coiled-coil alpha-helix.

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Abstract
CDNAs for laminin B chains have been isolated from a parietal endoderm cDNA library in pUC8 and pUC9. Identification is based on: ability to direct the synthesis in Escherichia coli of polypeptides carrying laminin antigen determinants, in vitro translation of hybrid selected mRNA, and hybridization to high mol. wt. RNA differentially expressed in cells synthesizing large amounts of laminin. The plasmid pPE9 hybrid selects mRNA for the B2 (mol. wt. 185 000) chain and provides 217 residues of C‐terminal amino acid sequence. The plasmids pPE386 and 49 both hybrid select mRNAs for the B1a (mol. wt. 205 000) and B1b (mol. wt. 200 000) chains. These two cDNAs are identical over much of their sequence, but pPE386 includes 133 nucleotides of 3′ non‐coding sequence and a poly(A) tail. Together they provide 495 residues of C‐terminal amino acid sequence. Analysis of the predicted sequences reveals a striking heptad repeat, with a high probability that residues a and d are hydrophobic. Such a repeat is typical of the coiled‐coil alpha‐helices found in proteins such as myosin, tropomyosin and desmin (2‐stranded) and fibrinogen (3‐stranded).