The polymorphic proteins TgrB1 and TgrC1 function as a ligand-receptor pair in Dictyostelium allorecognition

Abstract

Allorecognition is a key factor in Dictyostelium development and sociality. It is mediated by two polymorphic transmembrane proteins, TgrB1 and TgrC1, that contain extracellular immunoglobulin domains. TgrB1 and TgrC1 are necessary and sufficient for allorecognition and they carry out separate albeit overlapping functions in development, but their mechanism of action is unknown. Here we show that TgrB1 acts as a receptor and TgrC1 as its ligand in cooperative aggregation and differentiation. The proteins bind each other in a sequence-specific manner, TgrB1 exhibits a cell-autonomous function, and TgrC1 acts non-cell-autonomously. The TgrB1 cytoplasmic tail is essential for its function and it becomes phosphorylated upon association with TgrC1. Dominant mutations in TgrB1 activate the receptor function and confer partial ligand independence. These roles in development and sociality suggests that allorecognition is critical in the integration of individual cells into a coherent organism.