Novel Drosophila laminin A chain reveals structural relationships between laminin subunits

Laminins are trimeric glycoproteins composed of A, B1, and B2 chains that play important roles in cell adhesion and differentiation and in the assembly of basement membranes. There is evidence that multiple independent gene products can be used to supply the A, B1, and B2-like chains, thereby generating diverse laminin molecules. The set of A chain-like polypeptides are the most divergent. The amino-terminal portion of each chain, called the short arm, is composed of alternating cysteine-rich and globular domains. The amino-terminal two-thirds of the newly characterized Drosophila laminin A chain appears to encode a novel structure, but the analysis presented here shows that substantial portions of its amino acid sequence are related to sequences found in other A, B1, and B2 chains. A portion of the Drosophila A chain is composed of sequences like those found in B2 chains. These sequences parallel a portion of the vertebrate A chains, suggesting a structure for a shared ancestral A chain. However, unlike vertebrate A chains, the Drosophila A chain also contains sequences typical of B1 chains, suggesting that its gene must have arisen by recombination of segments from different primordial laminin genes.