Claudins function as major constituents of the tight junction complexes that regulate the permeability of epithelia. While some claudin family members function as impermeable barriers, others mediate the permeability to ions and small molecules. Often, several claudin family members are coexpressed and interact with each other, and this determines the overall permeability. CLDN15 forms tight junctions that mediate the paracellular transport of small monovalent cations along a concentration gradient, due to selective permeability for Na(+), Li(+) and K(+) ions, but selects against Cl(-) ions. Plays an important role in paracellular Na(+) transport in the intestine and in Na(+) homeostasis. Required for normal Na(+)-dependent intestinal nutrient uptake (By similarity). Can form linear homooligomers in the membrane, giving rise to tight junction strand-like structures. Tight junctions form continuous circumferential cell-cell contacts at the borders of apical and lateral cell membranes that seal the intercellular space and show up as strand-like structures in electron microscopy. Detected in kidney, jejunum and colon (at protein level). Palmitoylated. Belongs to the claudin family.