This family of proteins found in eukaryotes represents sperm acrosome-associated protein 9 (SPACA9, previously known as C9orf9 or MAST). Sperm acrosome-associated protein 9 has been suggested to form a complex with calcium-binding proteins calreticulin and caldendrin localised to the acrosome. Despite this, no known protein interaction motifs have been identified in MAST/SPACA9 [1].

1: A mouse protein that localizes to acrosome and sperm tail is regulated by Y-chromosome. Bhattacharya R, Devi MS, Dhople VM, Jesudasan RA; BMC Cell Biol. 2013;14:50. PMID:24256100