Characterization of Pepsin Fragments of Laminin in a Tumor Basement Membrane. Evidence for the Existence of Related Proteins

Abstract

Laminin was extracted from a [mouse] tumor basement membrane and subjected to extensive degradation by pepsin. The treatment released 2 homogenous fragments P1 (MW = 290,000) and P2 (MW = 45,000) in addition to a mixture of smaller peptides. Fragments P1 and P2 together contained > 90% of the disulfide bonds and accounted for .apprx. 1/3 of the mass of laminin. Both peptides differed in amino acid composition, immunological properties and a complex chain structure demonstrating the existence of 2 disulfide-bonded domains in the molecule. Part of the laminin in the tumor matrix could only be solubilized by pepsin treatment and several fragments were purified. The major fragment P1 closely resembled P1 of soluble laminin in its chemical and immunological properties. Minor fragments Pa, Pb, Pc and Pd (MW = 44,000-74,000) were only related to P1 or P2 in amino acid composition, chain pattern and antigenicity. Proteins similar but not identical to laminin exist in the basement membrane and account for the minor peptide variants.