STK33 Phosphorylates Fibrous Sheath Protein AKAP3/4 to Regulate Sperm Flagella Assembly in Spermiogenesis

Spermatogenesis defects are important for male infertility; however, the etiology and pathogenesis are still unknown. Herein, we identified two loss of function mutations of STK33 in 7 individuals with non-obstructive azoospermia. Further functional studies of these frameshift and nonsense mutations revealed that Stk33 male mice were sterile, and Stk33 sperm were abnormal with defects in the mitochondrial sheath (MS), fibrous sheath (FS), outer dense fiber (ODF) and axoneme. Stk33 male mice were subfertile and had oligoasthenozoospermia. Differential phosphoproteomic analysis and in vitro kinase assay identified novel phosphorylation substrates of STK33, fibrous sheath components AKAP3 and AKAP4, whose expression levels decreased in testis after deletion of Stk33. STK33 regulated the phosphorylation of AKAP3/4, affected the assembly of fibrous sheath in the sperm, and played an essential role in spermiogenesis and male infertility.