This family consists of several mammalian specific proacrosin binding protein sp32 sequences. sp32 is a sperm specific protein which is known to bind with with 55- and 53-kDa proacrosins and the 49-kDa acrosin intermediate. The exact function of sp32 is unclear, it is thought however that the binding of sp32 to proacrosin may be involved in packaging the acrosin zymogen into the acrosomal matrix [1].

1: An acrosomal protein, sp32, in mammalian sperm is a binding protein specific for two proacrosins and an acrosin intermediate. Baba T, Niida Y, Michikawa Y, Kashiwabara S, Kodaira K, Takenaka M, Kohno N, Gerton GL, Arai Y; J Biol Chem 1994;269:10133-10140. PMID:8144514