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 at low levels in skin. Phosphorylated. 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.