EXOTIC COLLAGEN GRADIENTS IN THE BYSSUS OF THE MUSSEL MYTILUS-EDULIS

Byssal threads of the common mussel Mytilus edulis contain collagenous molecules from which two pepsin-resistant fragments have been isolate d and characterized, These show a complementary distribution along the length of the thread, such that one predominates distally (Col-D) and the other proximally (Col-P). Both fragments contain three identical alpha-like chains with molecular masses of 50kDa (Col-P) and 60kDa (Co l-D) and have typically collagenous amino acid compositions; for examp le, 35% glycine and almost 20% proline plus 4-trans-hydroxyproline. Hy droxylysine and 3-hydroxyproline were absent. Col-P sequences are also typical of collagen in consisting of tandem repeats of the triplet GI SI-X-Y in which X and Y generally represent any amino acid, When proli ne occurs, it is hydroxylated to 4-trans-hydroxyproline only in the Y position, Seven instances where X is glycine have been detected in Col -P, Specific polyclonal anti-Col antibodies were used to isolate the p recursors of Col-P and Col-D from the mussel foot. PreCol-P has a mole cular mass of 95 kDa and contains 36% glycine but a lower imino acid c ontent (13%). It has a complementary distribution with another precurs or (preCol-D, 97 kDa) along the length of the foot, The two precursor compositions suggest resilin-like and silk-fibroin-like structures, re spectively, in the noncollagenous domains of preCol-P and preCol-D, Im munogold labelling studies indicate that Col-P is associated with the coiled fibers of the inner core in the proximal portion of the thread, whereas Col-D is localized to the straight fiber bundles of the dista l thread as well as to the outer core of the proximal thread.