Forms a triad with cellulose and pspB that is essential for spore outer layer formation. Binds to the C-terminal region of pspB. Depends on pspB for incorporation into the coat. Accumulates initially in vesicles distinct from prespore vesicles (PSVs). Expressed at or just prior to spore coat formation, at 20 to 22 hours of development of the fruiting bodies. Secreted to the cell surface before cellulose is deposited. The N-terminal part (18-235) contains the information required for insertion into the coat.