dimerization/docking (D/D) domain of the sperm fibrous sheath proteins, Calcium-Binding tYrosine-phosphorylation Regulated protein and Sperm Protein 17
CABYR and SP17 are naturally located in the human sperm fibrous sheath (FS). CABYR was originally isolated from spermatoza and was thought to be testis-specific, but has recently been observed in lung and brain tumors. It is a polymorphic calcium binding protein that is phosphorylated during capacitation. SP17 plays an important role in the interaction of sperm with the zona pellucida during fertilization. It also promotes cell-cell adhesion. SP17 is found in various human tumors of unrelated histological origin including metastatic squamous cell carcinoma, multiple myeloma, ovarian cancer, and primary nervous system tumors, among others. Both CABYR and SP17 contain an N-terminal dimerization/docking (D/D) domain with similarity to the D/D domain of the R subunit of cAMP-dependent protein kinase (PKA). The D/D domain of the R subunit dimerizes to form a four-helix bundle that serves as a docking site for A-kinase-anchoring proteins (AKAPs), which facilitates the localization of PKA to specific sites in the cell. The D/D domain of CABYR and SP17 have been shown to bind to AKAP3, a protein that is also associated to the FS of mammalian spermatozoa.
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