Biochemical characterization of Tractin and LeechCAM, two Ig-superfamily members involved in regulation of axonal outgrowth of Leech neurons
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Tractin and LeechCAM are two recently identified members of the Ig-superfamily. Tractin has a highly unusual domain organization: 6 Ig-like domains, 4 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. Our data show that Tractin is proteolytically processed into four fragments at two cleavage sites: one located in the third FNIII domain, and the other one located just proximal to the transmembrane domain. The most NH2-terminal fragment is shown to be glycosylated with the glycoepitopes of Lan3-2, Lan4-2 and Laz2-369, while the other remaining fragments are proposed to form a secreted homodimer and a transmembrane heterodimer. The PG/YG repeat region has sequence similarity to collagen and can be selectively digested by collagenase. It is shown by yeast two-hybrid analysis that the intracellular domain of Tractin can interact with ankyrin. Therefore, the various characteristics of Tractin may provide it with properties that link the extracellular matrix and the intracellular cytoskeleton together;LeechCAM is shown to be the leech homolog of apCAM, Fas II, and vertebrate NCAM by phylogenetic analysis. It has a domain configuration of five Ig-like domains, two FNIII-like domains, a transmembrane domain and a cytoplasmic domain. Our data show that LeechCAM probably exists in two forms, either with or without the transmembrane domain and the intracellular tail. Immunoprecipitation analysis shows that LeechCAM is glycosylated with the Laz2-369 glycoepitope, which has been specifically implicated in regulation of axonal outgrowth and synapse formation.