CATSPERG

Model summary

An auxiliary membrane subunit that organizes and stabilizes a sperm flagellar calcium-channel assembly. Its large extracellular region forms a multivalent binding scaffold that positions and maintains the channel array at the flagellar membrane, while its cytosolic tail anchors intracellular adaptors. By coupling adhesion-like and scaffolding functions to a single-pass membrane anchor, it supports calcium-dependent hyperactivation and the acrosome reaction in sperm and extends to broader roles in junctional organization and morphogenesis.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide starts with IPR053872 (CATSPERG, N-terminal domain, residues 44–203), followed by a large IPR053871 (CATSPERG, beta-propeller domain, residues 207–649), then an IPR053874 (CATSPERG, Ig-like domain, residues 753–879), and ends with IPR053873 (CATSPERG, C-terminal domain, residues 905–1106). The entire sequence is encompassed by IPR028246 (Cation channel sperm-associated auxiliary subunit gamma family, residues 14–1147). This ordered luminal ectodomain—N-terminal module, central beta-propeller, and Ig-like fold—preceding a single C-terminal domain strongly implies a type I single-pass membrane protein whose large extracellular region mediates adhesion-like and scaffolding interactions rather than catalysis. The presence of a beta-propeller and Ig-like domain creates multivalent binding surfaces that stabilize supramolecular assemblies and mediate extracellular contacts, which causally supports a non-enzymatic role centered on GO:0005515 protein binding.

2. 02

   From this binding-centric ectodomain, I infer the molecular function as protein binding (GO:0005515).

   The auxiliary-subunit family signature (IPR028246) ties this binding activity specifically to the CatSper calcium channel apparatus in sperm flagella. The N-terminal and beta-propeller modules likely nucleate assembly of the CatSper complex, while the Ig-like domain confers rigidity and spacing that tune the extracellular architecture of the flagellar membrane domain. The C-terminal cytosolic tail (IPR053873) would anchor intracellular partners and cytoskeletal adaptors, completing a bidirectional coupling that stabilizes the channel array.

3. 03

   This binding-driven architecture explains higher-order biology.

   By scaffolding and stabilizing the CatSper complex, the protein enables Ca2+-dependent hyperactivation and the acrosome reaction in sperm, processes that require tightly regulated membrane-channel assemblies. The same adhesive and organizational logic extends to epithelial and developmental contexts: Ig-like and beta-propeller modules commonly regulate junctional integrity and morphogenetic signaling. This provides a mechanistic route to GO:0045216 cell-cell junction organization and to morphogenetic programs such as GO:0001947 heart looping, where coordinated cell adhesion and signaling are essential.

4. 04

   Cellular location follows directly from the architecture and function.

   A single-pass topology with a large luminal ectodomain and cytosolic tail is characteristic of membrane-tethered scaffolds, placing the protein at the GO:0005886 plasma membrane. In sperm, this membrane residency is specialized to the flagellar principal piece, where CatSper arrays reside and where Ca2+ fluxes drive motility.

5. 05

   Mechanistically, I hypothesize that the N-terminal and beta-propeller domains assemble an extracellular lattice that positions and stabilizes CatSper channel subunits within the flagellar membrane, while the Ig-like domain enforces spacing and mechanical resilience.

   The C-terminal domain recruits intracellular adaptors and cytoskeletal regulators, coupling the channel complex to local signaling hubs. Likely partners include core CatSper pore-forming and auxiliary subunits, flagellar cytoskeletal elements, and junctional/cortical scaffolds that together coordinate Ca2+ entry with motility and exocytosis.