SignificanceCellular communication along the filaments of heterocyst-forming, nitrogen-fixing cyanobacteria has been discussed for at least 50 y but how this might be accomplished is not fully understood. We recently showed that the septum between heterocysts and vegetative cells is pierced by channels 12 nm in diameter and 20 nm long. Here, we show that three proteins, FraC, FraD, and FraG, participate in the formation of the channels although none of them appears to be a structural component of the channels. Moreover, using gold particle-labeled antibody, FraG was found around the cyanophycin plug as well as associated with the cytoplasmic membrane in the neighborhood of the peptidoglycan that forms the septum.