Laminin variants: why, where and when?

Laminin is a member of a family of proteins that are composed of three subunits, one heavy chain and two light chains. Five subunits in the laminin family have been cloned and sequenced so far. These include two heavy chains, the laminin A chain and the merosin M chain, and three light chains, B1, B2, and S. These five subunits can form four different laminin variants: A-B1-B2, A-S-B2, M-B1-B2, and M-S-B2, all having the B2 chain in common. Major basement membranes in tissues contain at least one of the four laminin variants. For example, the adult muscle and nerve basement membranes contain M-B1-B2, smooth muscle contains A-B1-B2, the myotendinous junction and the trophoblast basement membrane in the placenta contain M-S-B2, and blood vessels contain A-B1-B2 and/or A-S-B2. In the brain, the merosin M chain is present in association with neuronal fibers. The four members of the laminin family interact with cells in a similar manner. Thus, they promote outgrowth of neurites from neuronal cells and promote attachment and spreading of non-neuronal cells. The interaction of cells with laminins is mediated largely by integrin type receptors, including integrins alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, and alpha 6 beta 1. The expression of the different laminin-like proteins is developmentally regulated. The laminin A chain is the first heavy chain expressed.(ABSTRACT TRUNCATED AT 250 WORDS)