In Dictyostelium discoideum, TgrB1 and TgrC1 are partners of a heterophilic cell-adhesion system. To investigate its assembly process, the split GFP complementation assay was used to track the oligomeric status of both proteins. The ability of TgrC1 to form cis-homodimers spontaneously was demonstrated by fluorescence complementation studies and confirmed by chemical cross-linking. In contrast, TgrB1 failed to form cis-homodimers in the absence of TgrC1. Treatment of cell aggregates with antibodies against TgrB1 or TgrC1 did not affect TgrC1 dimerization, but inhibited TgrB1 dimer formation, suggesting that TgrB1 cis-homodimerization is dependent on trans-interaction with TgrC1. When TgrB1 and TgrC1 conjugated with the complementary halves of GFP were co-expressed in cells, cis-heterodimers were not detected. However, weak FRET signals were detected in cells expressing TgrB1–RFP and TgrC1–GFP, suggesting that TgrB1 dimers and TgrC1 dimers were arranged juxtapose to each other in the adhesion complex. The results of the present study suggest that the assembly process is initiated upon trans-interaction of monomeric TgrB1 with TgrC1 homodimers on adjacent cells, which triggers the formation of TgrB1 dimers. The homodimerization of TgrB1 in turn induces the clustering of TgrB1 and TgrC1, and the coalescence of TgrB1–TgrC1 clusters results in the formation of large adhesion complexes.

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