When phosphorylated, plays a role in filament reorganization. Involved in the delivery of mutated CFTR to the plasma membrane. Involved in the uptake of thrombin-antithrombin complexes by hepatic cells (By similarity). Together with KRT8, is involved in interleukin-6 (IL-6)-mediated barrier protection. Heterotetramer of two type I and two type II keratins. KRT18 associates with KRT8 (PubMed:24940650). Interacts with PLEC isoform 1C, when in a heterodimer with KRT8 (PubMed:24940650). Interacts with PNN and mutated CFTR. Interacts with YWHAE, YWHAH and YWHAZ only when phosphorylated. Interacts with the thrombin-antithrombin complex. Interacts with DNAJB6, TCHP and TRADD (By similarity). Interacts with FAM83H (By similarity). Interacts with EPPK1 (PubMed:25617501). Expressed in endoderm, intestinal epithelial cells and in most extraembryonic tissues. During embryogenesis, expressed in a complex spatial and temporal pattern in various embryonic epithelia. In 7.5 and 13.5 day old embryo, expressed in most endodermal epithelia, ectodermal and nascent mesodermal tissues. When the neural plate forms, expression begins in the cells of skin ectoderm, head process/notochord, periderm, whisker buds, choroid plexus and the epithelia of auditory duct and inner ear. High expression in the lining endodermal cells when the foregut and hindgut invaginations form. Expression in all three layers of the urothelium starts at day 15 in the embryo and is not visible after day 18. By day 11 and 12, the entire embryonic palatal epithelium shows expression as well as the nasal passages and the roof of the mouth; which disappears progresively from day 13 to 15. By retinoic acid and IL-6. Phosphorylation increases by IL-6. Proteolytically cleaved by caspases during epithelial cell apoptosis. Cleavage occurs at Asp-231 by either caspase-3, caspas-6 or caspase-7. O-GlcNAcylation increases solubility, and decreases stability by inducing proteasomal degradation. 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.