Merosin is a basement membrane-associated protein found in placenta, striated muscle, and peripheral nerve. A 3.6-kilobase merosin cDNA clone was isolated from a placental cDNA expression library. The clone contained a 3.4-kilobase open reading frame, the 3' portion of which includes protein sequences of proteolytic fragments of merosin. The deduced amino acid sequence of the merosin polypeptide was similar to that of the COOH-terminal region of the 400-kDa A chain of laminin. This part of laminin forms the large globule at the end of the long arm of the laminin cross and is thought to contain the neurite-promoting site and the major cell binding site(s) in laminin. The sequence identity between merosin and the laminin A chain in this region is nearly 40%.An antiserum against a synthetic peptide from the middle ofthe merosin cDNA sequence identified a 300-kDa polypeptide in placental extracts, indicating that the merosin polypeptide is similar in size to the laminin A chain. Intact merosin was isolated from placental extracts and shown to be covalently associated with the laminin B chains and to have a cross-like structure similar to that of laminin. The similarities between merosin and laminin show that both proteins are members of the same family of basement membrane proteins.Merosin is a tissue-restricted basement-membrane protein (1). It is present in the trophoblast basement membrane of the human placenta and in basement membranes of striated muscle and Schwann cells. In the mouse, merosin is not present in the fetus and only appears in muscle and nerve after birth (1). This late developmental appearance suggests a role for merosin in the maturation or differentiation of tissues. These results parallel the limited expression of merosin in human malignant tumors (2, 56) and its absence in cultured cell lines (1), which are at a lower level of differentiation than their normal tissue counterparts. The interesting spatial and temporal expression of merosin prompted us to determine its structure as a step toward elucidating its function. As shown here, merosin is structurally related to laminin.In contrast to merosin, laminin is thought to be present in all basement membranes (3). It is a large glycoprotein composed of three polypeptide chains, a 400-kDa heavy (A) chain and two light (B) chains of about 200 kDa. The entire heavy chain of mouse laminin (4, 5) and part of the human laminin heavy chain (6) have been sequenced. The B1 and B2 light chains from mouse (7-9, 57), human (10, 11), and Drosophila (12, 13) have also been sequenced. The NH2-terminal twothirds of the A chain are homologous to the B1 and B2 chains but the COOH-terminal one-third has a distinct structure.Laminin promotes attachment, spreading, motility, and growth of a variety of cell types (14)(15)(16)(17)(18). One of the most striking activities of laminin is its capacity to promote outgrowth of neurites from cultured neuronal cells (19)(20)(21). A major site of cell adhesion and the neurite-promoting activity appears to reside in or...

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