Functions in biogenesis and organization of the apical membrane of epithelial cells of the thick ascending limb of Henle's loop (TALH), where it promotes formation of complex filamentous gel-like structure that may play a role in the water barrier permeability. May serve as a receptor for binding and endocytosis of cytokines (IL-1, IL-2) and TNF. Facilitates neutrophil migration across renal epithelia. In the urine, may contribute to colloid osmotic pressure, retards passage of positively charged electrolytes, prevents urinary tract infection and inhibits formation of liquid containing supersaturated salts and subsequent formation of salt crystals. Homodimer that then polymerizes into long filaments. The filaments can additionally assemble laterally to form a sheet. Only a small fraction sorts to the basolateral pole of tubular epithelial cells compared to apical localization. Secreted into urine after cleavage. Colocalizes with NPHP1 and KIF3A. Detected in urine. The ZP domain mediates polymerization, leading to the formation of long filaments. The core of the filament consists of stacked ZP domains which assemble into a helical structure. Each ZP domain consists of an N-terminal (ZP-N) and C-terminal (ZP-C) region connected by a flexible linker; the linker allows the ZP domain to wrap around the ZP-C subdomain of the preceding subunit. The heavily glycosylated N-terminal part of the protein (containing several EGF-like domains) forms branches which protrude from the core and may be involved in pathogen capture. N-glycosylated. Proteolytically cleaved at a conserved C-terminal proteolytic cleavage site to generate the secreted form found in urine. This cleavage is catalyzed by HPN.