Filamentous cyanobacteria produce strings of cells, most of which are photosynthetic vegetative cells with larger nitrogen-fixing heterocyst, spaced in the filament from each other by 10 - 20 vegetative cells.  All are connected by intercellular septal channels or pores that probably consist of several proteins.  Three of these have been identified, SepJ (TC# 2.A.7.3.81) as well as FraC (P46078) and FraD (P46079), listed in Family 1.V.1.