BON1 belongs to a poorly characterized family of proteins called the copines. Like other members of this family it has a calcium-stimulated phospholipid binding activity, and can enhance vesicle aggregation in vitro. Little else is known about copines, but the binding of one to secretory vesicles, and the localization of BON1 to the plasma membrane have led to suggestions that copines participate in exocytosis.
The expression of BON1 in growing tissues, and its increased expression at low temperatures, suggest that BON1 could compensate for otherwise inefficient membrane fusion at low temperature. “It makes sense having this hypothesis,” says Hua, “but there is no direct evidence.” ▪