Laminin

Laminins are the major non-collagenous component of the basal lamina, such as those on which cells of an epithelium sit. They are a family of glycoproteins that are an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Laminins are secreted and incorporated into cell-associated extracellular matrices. They are shaped like a cross.

Types
Each laminin molecule is a heterotrimer assembled from alpha-, beta-, and gamma-chains.


 * There are five forms of alpha-chains:, , , ,


 * There are four of beta-chains:, , ,


 * There are three of gamma-chains:, ,

Fifteen laminin trimers have been identified.

Networks
Laminins form independent networks and are associated with type IV collagen networks via entactin, and perlecan. They also bind to cell membranes through integrin receptors and other plasma membrane molecules, such as the dystroglycan glycoprotein complex and Lutheran blood group glycoprotein. Through these interactions, laminins critically contribute to cell attachment and differentiation, cell shape and movement, maintenance of tissue phenotype, and promotion of tissue survival. Some of these biological functions of laminin have been associated with specific amino-acid sequences or fragments of laminin. For example, the peptide sequence [GTFALRGDNGDNGQ], which is located on the alpha-chain of laminin, promotes adhesion of endothelial cells.

Pathology
Dysfunctional structure of one particular laminin, laminin-2, is the cause of some forms of muscular dystrophy. Laminin-2 is composed of an α2, a β1 and a γ1 chains. This laminin's distribution includes the brain and muscle fibers. In muscle, it binds to alpha dystroglycan via the G domain, and via the other end binds to the extracellular matrix. Progeria is the result of a defect in the prolaminin receptor that prevents the cleaving and activation of prolaminin into laminin.