Proteolytic cleavage of the ectodomain of the L1 CAM family member Tractin.
Tractin is a member of the L1 family of cell adhesion molecules in leech. Immunoblot analysis suggests that Tractin is constitutively cleaved in vivo at a proteolytic site with the sequence RKRRSR. This sequence conforms to the consensus sequence for cleavage by members of the furin family of convertases, and this proteolytic site is shared by a majority of other L1 family members. We provide evidence with furin-specific inhibitor experiments, by site-specific mutagenesis of Tractin constructs expressed in S2 cells, as well as by Tractin expression in furin-deficient LoVo cells that a furin convertase is the likely protease mediating this processing. Cross-immunoprecipitations with Tractin domain-specific antibodies suggest that the resulting NH(2)- and COOH-terminal cleavage fragments interact with each other and that this interaction provides a means for the NH(2)-terminal fragment to be tethered to the membrane. Furthermore, in S2 cell aggregation assays we show that the NH(2)-terminal fragment is necessary for homophilic adhesion and that cells expressing only the transmembrane COOH-terminal fragment are non-adhesive. However, tethering of exogeneously provided Tractin NH(2)-terminal fragment to S2 cells expressing only the COOH-terminal fragment can functionally restore the adhesive properties of Tractin.
Xu
Ying-Zhi
YZ
Department of Zoology and Genetics, Iowa State University, Ames 50011, USA.
Ji
Yun
Y
Zipser
Birgit
B
Jellies
John
J
Johansen
Kristen M
KM
Johansen
Jorgen
J
D000818:Animals; D015153:Blotting, Western; D015816:Cell Adhesion Molecules, Neuronal / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D004591:Electrophoresis, Polyacrylamide Gel; D045683:Furin; D006868:Hydrolysis; D007865:Leeches; D016297:Mutagenesis, Site-Directed; D013381:Subtilisins / Q000378:metabolism
D015816:Cell Adhesion Molecules, Neuronal; C107284:tractin protein, Hirudo medicinalis; D013381:Subtilisins; D045683:Furin