The structure and organization of the bile canalicular cytoskeleton with special reference to actin and actin-binding proteins.

The distribution of actin filaments and actin-binding proteins in the bile canaliculus (BC) of normal human hepatocytes was determined as a means of establishing the structure and organization of the BC cytoskeleton. Immunoblots demonstrated that actin, and the actin-binding proteins, myosin II, tropomyosin, vinculin, alpha-actinin, villin, were present, as were the non-actin-related proteins beta-tubulin, and cytokeratins. Three actin filament regions were identified: microvillus core filaments, a membrane-associated microfilamentous network, and a circumferential pericanalicular actin filament band. Actin-binding proteins were nonrandomly associated with actin in these regions. In the case of the pericanalicular band, there was also association with the zonula adherens junction. Intermediate filaments inserted into desmosomes. The ultrastructural localization of the actin-binding proteins was fundamentally linked to the arrangement and organization of the major canaliculus-associated microfilament structures. Structural organization of the cytoskeleton was also linked to distinct components of the intercellular junctions. It is notable that tropomyosin and a-actinin, which in muscle cells are regulatory proteins of contractile activity, and myosin II are associated with the pericanalicular actin microfilament band; it is the BC counterpart of the contractile actin filament band found in the apical region of other secretory cells. The outer sheath of noncontractile intermediate filaments likely stabilizes the canalicular compartment.