IGF-binding proteins prolong the half-life of the IGFs and have been shown to either inhibit or stimulate the growth promoting effects of the IGFs on cell culture. They alter the interaction of IGFs with their cell surface receptors. Also exhibits IGF-independent antiproliferative and apoptotic effects mediated by its receptor TMEM219/IGFBP-3R. Promotes testicular germ cell apoptosis. Interacts with XLKD1. Binds IGF2 more than IGF1. Forms a ternary complex of about 140 to 150 kDa with IGF1 or IGF2 and a 85 kDa glycoprotein (ALS). Interacts with TMEM219 (By similarity). Phosphorylated by FAM20C in the extracellular medium. No effect on baseline apoptosis in the testis but germ cell apoptosis is dramatically reduced following treatment with a gonadotropin-releasing hormone antagonist.