Contributions of molecular motor enzymes to vesicle-based protein transport in gastrointestinal epithelial cells  Mark A. McNiven, Kimberly J. Marlowe  Gastroenterology  Volume 116, Issue 2, Pages 438-451 (February 1999) DOI: 10.1016/S0016-5085(99)70142-3 Copyright © 1999 American Gastroenterological Association Terms and Conditions

Fig. 1 General organization of the actin and microtubule. Cytoskeleton and associated motor enzymes. (A) Orientation of microtubules and actin in a generic nonpolarized epithelial cell. Microtubules (dark lines) extend from a perinuclear centrosome with plus or rapidly assembling ends protruding into the peripheral cytoplasm. Actin filaments (thin lines) may be organized into large parallel stress fibers or as a subcortical meshwork. (B) Similarities and differences between the two major classes of microtubule-based motor enzymes. Both kinesin and dynein interact with the microtubule surface lattice via large globular head domains that also contain ATP-binding sites. Interaction of each motor with a membrane vesicle is believed to be mediated by associated light or intermediate chains, which are shaded and attached to the vesicle. Note that kinesin translocates toward a microtubule plus end while dynein moves in the opposite direction toward the minus end. (C). Diagrammatic representation of two types of “tailless” nonmuscle myosins. Like the microtubule-based enzymes, both myosins interact with an actin filament and hydrolyze ATP via a globular head domain. Although myosin I consists of a single head with a modest-sized tail, myosin V is a dimeric enzyme with an exceptionally large tail. How these myosin molecules interact with the surface of a lipid vesicle is undefined. (D) The putative assembly, scission, and disassembly states of dynamin during the liberation of a nascent vesicle from a donor compartment. Dynamin monomers are believed to self-associate into tetramers that assemble about the neck of a membrane invagination. Upon GTP hydrolysis, the dynamin polymer may undergo a conformational change to constrict and sever the membrane. Gastroenterology 1999 116, 438-451DOI: (10.1016/S0016-5085(99)70142-3) Copyright © 1999 American Gastroenterological Association Terms and Conditions

Fig. 2 Distribution of a microtubule-based motor, kinesin, in cultured hepatocytes and acinar cells. Immunostaining of (A) a primary hepatocyte and (B) acinar cell labeled with a polyclonal antibody to the KHC. Strong labeling of the Golgi apparatus, situated about the canaliculus (arrow), is apparent in the hepatocyte, and periluminal zymogen granules are stained in the acinar cell. (C and D) Hepatocyte– and acinar cell–derived cell lines stained with the same kinesin antibodies as in A and B shows prominent labeling of punctate vesicles (arrowheads) and the Golgi apparatus (arrow). Gastroenterology 1999 116, 438-451DOI: (10.1016/S0016-5085(99)70142-3) Copyright © 1999 American Gastroenterological Association Terms and Conditions

Fig. 3 Cytoplasmic vesicles bind to cytoskeletal filaments in vitro. (A–C) Electron microscopic visualization of hepatocyte-derived membrane vesicles binding to Taxol-stabilized microtubules in the (A and B) absence or (C) presence of Mg:ATP. Note that nucleotide induces dissociation of vesicles from the cytoskeletal substrate. (D) Electron micrograph of membrane vesicles purified from epithelial cells of the intestinal brush border in the absence of Mg:ATP. Vesicles show a tight association with both microtubules (*) and actin filaments (arrowheads) under these nucleotide-free conditions. Gastroenterology 1999 116, 438-451DOI: (10.1016/S0016-5085(99)70142-3) Copyright © 1999 American Gastroenterological Association Terms and Conditions

Fig. 4 Organization of actin and tubulin in hepatocytes and pancreatic acinar cells. (A and B) In both of these cell types, microtubules appear to emanate from the apical membrane (arrows), which constitutes either a canalicular or luminal domain. Like microtubules, actin filaments are also intimately associated with the apical membrane as shown (C and D) by fluorescence microscopy of rhodamine-labeled phalloidin in fixed cells and (E and F) by high-magnification electron micrographs. Arrowheads point to bundles of actin filaments. N, nucleus; C, canaliculus; L, lumen. Green fluorescence staining in D represents zymogen granules labeled with a specific fluorescent marker antibody. Gastroenterology 1999 116, 438-451DOI: (10.1016/S0016-5085(99)70142-3) Copyright © 1999 American Gastroenterological Association Terms and Conditions

Fig. 5 Molecular motor enzymes mediate vesicle trafficking in GI epithelial cells: variations of a central theme. Some of the predicted functions for actin and microtubule-based motor enzymes in the hepatocyte and pancreatic acinar cell are shown. Arrowheads indicate predicted direction of vesicle translocation, and colors refer to the type of motor that may be used by a specific event. Note that the position of the Golgi apparatus in the hepatocyte is different from that in the acinar cell and enterocyte, suggesting that different molecular motor enzymes may be used. Although the overall organization of the acinar cell and the enterocyte is similar, there are several important distinctions. These include numerous prominent actin bundles filling the large microvilli in the enterocyte. Most importantly, the molecular motors in the enterocyte are engaged largely in transporting endocytosed material from the apex for excretion to the basolateral domain either directly or through endoplasmic reticulum (ER) via the formation of chylomicrons. In contrast, the acinar cell is focused on the transport and secretion of nascent zymogens to the apical domain, and the hepatocyte actively secretes and endocytoses at both domains. Gastroenterology 1999 116, 438-451DOI: (10.1016/S0016-5085(99)70142-3) Copyright © 1999 American Gastroenterological Association Terms and Conditions