The carboxy-terminal cysteine of the tetraspanin L6 antigen is required for its interaction with SITAC, a novel PDZ protein

PDZ domains are protein modules that mediate protein-protein interactions. Here, we present the identification and characterization of a protein simil ar to the recently identified PDZ-containing protein TACIP18, which we have named SITAC (similar to TACIP18). SITAC is preferentially expressed in cel ls of the digestive tract, associated with intracellular membranes. Despite the high degree of sequence identity between the PDZ domains of TACIP18 an d those of SITAC, none of the known Ligands of the former shows interaction with the latter, as judged by two-hybrid analysis. SITAC interacts with pe ptides containing bulky hydrophobic amino acids two positions upstream of t he C-terminal residue. Surprisingly, SITAC also shows interaction with pept ides ending in C, a previously unacknowledged ability of PDZ domains. The s equence -Y-X-C-COOH, bound in vitro by SITAC, is present in the member of t he tetraspanin superfamily, the L6 antigen. Coimmunoprecipitation experimen ts show that SITAC interacts with L6A, but not with an L6A C-terminal mutan t, confirming the capacity of SITAC to interact with proteins ending in C. Confocal analysis shows that the interaction between L6A and SITAC is neces sary for the precise colocalization of both molecules in the same subcellul ar compartment. In summary, the characterization of the protein SITAC has u nveiled novel sequences recognized by PDZ domains, and it suggests that L6A is a natural ligand of this PDZ protein.