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 simple epithelia such as intestinal mucosa, bile duct, hepatocytes, renal tubules, endothelia, ocular lens epithelium, and in a variety of mesenchymally-derived cells such as blood vessel endothelia, pillar gill cells, optic nerve glial cells, fibroblasts, interstitial cells, chondrocytes and ovarian theca cells. Also expressed in epidermis, pharyngeal mucosa, mucosa of anterior esophagus, gill mucosa and cornea. Proteolytically cleaved by caspases during epithelial cell apoptosis. 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.