STRUCTURAL STUDIES OF CONNECTIVE TISSUES

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The unique components intrinsic to basement membranes have been characterized, but the 3-dimensional organization of type IV collagen and its structural relation to other components remain to be defined. X-ray diffraction patterns of dried, stretched bovine lens capsule show, in addition to high angle reflections from the type IV triple-helix, series of 4.6nm and 33.5 nm meridional reflections which suggest a surprising degree of supramolecular order. We propose to clarify the relation between the x- ray data and native basement membrane structure. The origin of the observed repeating units in specific tissue components will be identified through x-ray studies on selectively extracted lens capsules and on individual components. Electron microscopy will be done on stretched and unstretched lens capsules, prepared by replica plating on freeze etched specimens, and optical diffraction of electron micrographs will be compared with x-ray diffraction patterns. Structural information on lens capsule, which represents a highly ordered and type IV rich basement membrane, will provide a basis for understanding the structure and function of other basement membranes and alterations in diseased states. Interpretation of our basement membrane x-ray data requires a model for the molecular conformation of type IV collagen. The classic triple-helical conformation found in fibrillar collagens has the stringent amino acid constraint that every third residue must by glycine, and Gly substitution have been associated with connective tissue diseases. In contrast, native type IV collagen contains more than 20 breaks in the repeating tripeptide (X-Y-Gly)n pattern. We hypothesize: (1) the effect of an interruption depends upon its general character [Gly substitution, Gly deletion, Y (or X) deletion, or insertion]; and (2) charged pair sequences surrounding an interruption lessen the structural consequences by allowing the peptide chain to loop out at the site. These hypotheses will be tested by characterizing the conformation, stability and folding defined sequence triple-helical peptides, including peptides with interruptions flanked by charged triplets, using circular dichroism and NMR spectroscopy, together with x-ray diffraction and computer modeling. Following studies on simple models, a peptide with a 34 residue type IV sequence which includes an interruption will be characterized, and 2-dimensional NMR will be used to determine the 3-dimensional conformation of a "designed" triple-helical peptide in solution. Results from these peptide studies will help define perturbations to the type IV triple-helix, and clarify the structural consequences of fibrillar collagen mutations.