When phosphorylated, plays a role in filament reorganization. Heterotetramer of two type I and two type II keratins. Keratin-18 associates with keratin-8 (By similarity). Expressed in all tissues examined, including epidermal keratinocytes, tongue mucosa, corneal epithelium, Meninges cells, epidermal goblet and Merkel cells, gill mucosa, basal cells of gill secondary lamellae, hepatocytes and bile canaliculi, bile duct, intestinal mucosa, blood vessel endothelial and muscle cells, interstitial cells, fibroblasts, scale-associated cells, ovary stroma and follicle, optic nerve glia, eye ciliary body, chondrocytes and kidney tubules. Phosphorylated. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.