Klotho is a transmembrane glycoprotein encoded by the KLOTHO gene and structurally characterized by a short intracellular domain and a large extracellular region containing two internal repeat domains, KL1 and KL2. These domains share sequence homology with family 1 glycosidases, although classical glycosidase enzymatic activity has not been clearly demonstrated. Post-translational processing by membrane-associated proteases such as ADAM10 and ADAM17 enables the extracellular domain of Klotho to be released into circulation and biological fluids, generating soluble forms that exhibit distinct biochemical properties from the membrane-bound protein.
At the molecular level, transmembrane Klotho functions as a co-receptor for fibroblast growth factor 23 (FGF23), contributing to phosphate and vitamin D homeostasis. Soluble Klotho has been reported to influence multiple signaling cascades through glycan modification of cell-surface proteins, including ion channels and growth factor receptors. These interactions suggest a regulatory role in pathways linked to oxidative stress responses, autophagy-lysosomal dynamics, insulin-related signaling, and cellular metabolism.
