Characterization of bovine anterior-lens-capsule basement-membrane collagen. 2. Segment-long-spacing precipitates: further evidence for large N-terminal and C-terminal extensions.

Collagen extracted from bovine anterior lens capsule basement membrane under minimally degradative conditions can be precipitated in the form of segment-long-spacing crystallites. Positive and negative stain preparations of these precipitates, using uranyl acetate, show the crystallites to have an asymmetric band pattern, indicating unidirectional orientation of all molecules in each crystallite. Non-banded globular masses appear at both ends of most of the crystallites, giving the segment-long-spacing aggregates the form of dumbells, with a total length of 300 nm. Repeated pepsin digestion removes first one then both globular regions leaving the helical region intact but with a length still equal to 300 nm. It thus appears that the pepsin-susceptibile protein is folded back over and covers the ends of the helical regions. Preparations with the most intact end regions tend to aggregate in fibrous segment-long-spacing form, that is, in end-interacted strings or nets. The very prominent molecular end-region interaction suggests a similar filamentous but non-periodic organization within native basement membrane.