When phosphorylated, plays a role in filament reorganization. Involved in the delivery of mutated CFTR to the plasma membrane. Together with KRT8, is involved in interleukin-6 (IL-6)-mediated barrier protection (By similarity). Involved in the uptake of thrombin-antithrombin complexes by hepatic cells. Heterotetramer of two type I and two type II keratins. KRT18 associates with KRT8 (By similarity). Interacts with PNN and mutated CFTR. Interacts with YWHAE, YWHAH and YWHAZ only when phosphorylated. Interacts with DNAJB6, TCHP and TRADD (By similarity). Interacts with the thrombin-antithrombin complex. Interacts with FAM83H (By similarity). Interacts with EPPK1 (By similarity). Expressed on the plasma membrane of hepatocytes and in the narrow apical portions of supporting cells in the vomeronasal sensory epithelium. Detected in the type III alveolar cells of the lung, in the proliferative crypt epithelium of the small intestine and in the older intragemmal cells of the tongue. Expression first detected in the male gonad at 13.5 dpc, at the onset of testicular differentiation, and at 17 dpc in cell aggregates of the early ovary; then only in some cord cells of the older ovary. Expressed in fetal lung epithelium at 20 dpc. Detected at 13 dpc, sparsely distributed throughout the cytoplasm in the single layer of periderm cells covering the dorsal epithelium of the fetal tongue. Expression increases in the lingual periderm cells at 17 dpc and then disappears at 19 dpc coinciding with the disappearance of the periderm cells at the onset of squamous stratification of the lingual epithelium. Expressed at day 2-3 postnatally in older, elongated taste bud cells and at day 5, uniformly distributed throughout the epithelium of villi, intervillus epithelium and developing crypt buds of the small intestine. By IL-6 (By similarity). By fibronectin. Phosphorylation increases by IL-6. Proteolytically cleaved by caspases during epithelial cell apoptosis. Cleavage occurs at Asp-231 by either caspase-3, caspase-6 or caspase-7 (By similarity). Dephosphorylated by ethanol. 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.