PRIMARY STRUCTURE OF THE HUMAN LAMININ A-CHAIN - LIMITED EXPRESSION IN HUMAN TISSUES

cDNA clones for the human laminin A chain were isolated from libraries prepared from human gestational choriocarcinoma cell line (JAR) RNA. They cover approx. 8 kb from the 5'-end of the 9.5 kb mRNA coding for this protein. Our clones contain 94 nucleotide residues for the 5'-end untranslated region and 7885 nucleotide residues of coding sequence. The complete human laminin A chain contains a 17-amino acid-residue signal peptide and a 3058-residue A chain proper. The human laminin A chain has a distinct domain structure with numerous internal cysteine-rich repeats. The large globular domain G has five repeats, which have several conserved glycine and cysteine residues. Furthermore the A chain contains 20 internal cysteine-rich repeats present in tandem arrays in three separate clusters (domains IIIa, IIIb and V). Domain I + II has a predicted continuous alpha-helical structure characterized by heptad repeats and three domains (IVa, IVb and VI) are predicted to contain a number of beta-sheets and coiled-coil structures. Northern-blot analysis was used to study the laminin A chain expression in the JAR cell line, full-term placenta and newborn-human tissues (kidney, spleen, lung, heart muscle, psoas muscle and diaphragm muscle). The expression was detectable in newborn-human kidney and JAR cell line only. The overall amino acid sequence identity between human and mouse is 76%. The human chain has only one Arg-Gly-Asp (RGD) sequence, which is located in the long arm within domain G, whereas the single RGD sequence in the mouse chain is located in the short arm in domain IIIb. The degree of identity between the human laminin A chain sequence and the sequence available for merosin [Ehrig, Leivo, Argraves, Ruoslahti & Engvall (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3264-3268] is about 41 % and when conservative substitutions are included the degree of similarity is 54 %.