Posttranslational processing and differential glycosylation of Tractin, an Ig-superfamily member involved in regulation of axonal outgrowth

Tractin is a novel member of the Ig-superfamily which has a highly unusual structure. It contains six Ig domains, four FNIII-like domains, an acidic domain, 12 repeats of a novel proline- and glycine-rich motif with sequence similarity to collagen, a transmembrane domain, and an intracellular tail with an ankyrin and a PDZ domain binding motif. By generating domain-specific antibodies, we show that Tractin is proteolytically processed at two cleavage sites, one located in the third FNIII domain, and a second located just proximal to the transmembrane domain resulting in the formation of four fragments. The most NH(2)-terminal fragment which is glycosylated with the Lan3-2, Lan4-2, and Laz2-369 glycoepitopes is secreted, and we present evidence which supports a model in which the remaining fragments combine to form a secreted homodimer as well as a transmembrane heterodimer. The extracellular domain of the dimers is mostly made up of the collagen-like PG/YG-repeat domain but also contains 11/2 FNIII domain and the acidic domain. The collagen-like PG/YG-repeat domain could be selectively digested by collagenase and we show by yeast two-hybrid analysis that the intracellular domain of Tractin can interact with ankyrin. Thus, the transmembrane heterodimer of Tractin constitutes a novel protein domain configuration where sequence that has properties similar to that of extracellular matrix molecules is directly linked to the cytoskeleton through interactions with ankyrin.