Type IV collagen was solubilized from a tumor basement membrane either by acid extraction or by limited digestion with pepsin. The two forms were similar in composition and the size of the constituent chains but differed when examined by electron microscopy and in the fragment pattern produced by bacterial collagenase. The acid-soluble form showed after rotary shadowing strands mainly of a length of 320 nm which terminated in a globule, or two strands connected by a similar globule. The globule was identified as a non-collagenous domain (NC1) which under dissociating conditions could be separated into two peptides showing a monomer-dimer relationship. Higher aggregates of NCI were visualized under non-dissociating conditions. Some of the acid-extracted molecules have retained the previously described 7-S collagen domain. The pepsin-solubilized form lacked domain NCI and consisted mainly of four triple-helical strands (length 356 nm) joined together at the 7-S domain (length 30 nm). Common to both forms of type IV collagen was a small collagenase-resistant domain NC2 which was composed of collagenous and non-collagenous elements and located between the 7-S domain and the major triple helix. These data indicate that the collagenous matrix of basement membranes consists of a regular network of type IV collagen molecules which is generated by two different interacting sites located at opposite ends of each molecule. The 7-S collagen domain connects four molecules while the NCI domain connects two molecules. The maximal distance between identical cross-linking sites (7-S or NCI) was estimated to be about 800 nm comprising the length of two molecules.Type IV collagen is a unique member among the collagenous proteins and is considered to be the major structural component of basement membranes [I 1. Biosynthetic studies have shown that the constituent chains of type IV collagen ( M , about 180000) are larger than those of interstitial collagens and procollagens, and that they are not substantially processed when deposited in the matrix [2-51. A further unique feature are frequent interruptions of the triple helix as indicated by sequence analysis [6]. This explains the protease sensitivity of native type IV collagen [7,8] and possibly causes a greater flexibility of the large triple-helical segments. These molecules also possess another short triple-helical segment (7-S domain) which appears to be part of a rather compact fragment named 7-S collagen [9,10].Electron microscopical studies have demonstrated that basement membranes have a rather amorphous appearance [I I], quite different from the cross-striated fibrillar structures of collagenous proteins observed in interstitial connective tissue. X-ray diffraction studies of stretched lens capsules have indeed indicated a poorly ordered fibrillar array [12]. On the basis of these observation Kefalides [I] suggested a model for the basement membrane matrix which envisions sheets of type IV collagen to be cross-linked to alternating layers of non-collagenous proteins. Ot...

Intima collagen was obtained from pepsin digests of human placenta in two forms, which differ to some extent in the size of their constituent polypeptide chains (Mr 50 000-70 000). These chains are connected by disulphide bonds to large aggregates. The aggregates are arranged in a triple-helical conformation with a remarkably high thermal stability (Tm 41-62 degrees C) and are resistant to further proteolytic digestion. Reduction of as little as 5% of the disulphide bonds produces mainly monomeric triple helices (Mr about 160 000) with Tm 32 degrees C. Partially reduced material can be separated into triple-helical and non-collagenous domains by proteolysis. Pepsin releases a collagenous component with chains of Mr 38 000. Bacterial collagenase liberates two non-collagenous segments (Mr 15 000-30 000) rich in cystine. Treatment with collagenase before reduction separates intima collagen into a large fragment composed of collagenous (Tm 41 degrees C) and non-collagenous structures and a single non-collagenous segment. The data support the electron-microscopical model of intima collagen [Furthmayr, Wiedemann, Timpl, Odermatt & Engel (1983) Biochem. J. 211, 303-311], indicating that the basic unit of the fragment consists of a continuous triple helix joining two globular domains.

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