May contribute to the structure of the coat at the interface between the middle, cellulosic layer and the outer, electron-dense, proteinaceous layer. Accumulates specifically in regulated secretory vesicles of prespore cells (prespore vesicles). The protein later accumulates extracellularly in the spore coat. By cAMP. Disulfide bonding is important for associating SP87 with the coat. Phosphorylated on serine residues.