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). Abundantly expressed in an even distribution throughout the optic nerve, localizing specifically to the astrocyte domains. Moderately expressed in spinal cord, brain, liver and oocytes. 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.