Structural and immunological characterization of the myosin-like 110-kD subunit of the intestinal microvillar 110K-calmodulin complex: evidence for discrete myosin head and calmodulin-binding domains

Characterization of intestinal microvillar membrane disks: detergent-resistant membrane sheets enriched in associated brush border myosin I (110K-calmodulin)

The actin bundle within each microvillus of the intestinal brush border (BB) is tethered laterally to the membrane by bridges composed of BB myosin I. Avian BB myosin I, formerly termed 110K-calmodulin, consists of a heavy chain with an apparent Mr of 110 kD and three to four molecules of calmodulin "light chains." Recent studies have shown that this complex shares many properties with myosin including mechanochemical activity. In this report, the isolation and characterization of a membrane fraction enriched in bound BB myosin I is described. This membrane fraction, termed microvillar membrane disks, was purified from ATP extracts of nonionic detergent-treated microvilli prepared from avian intestinal BBs. Ultrastructural analysis revealed that these membranes are flat, disk-shaped sheets with protrusions which are identical in morphology to purified BB myosin I. The disks exhibit actin-activated Mg-ATPase activity and bind and cross-link actin filaments in an ATP-dependent fashion. The mechanochemical activity of the membrane disks was assessed using the Nitella bead movement assay (Sheetz, M. P., and J. A. Spudich. 1983. Nature [Lond.]. 303:31-35). These preparations were shown to be free of significant contamination by conventional BB myosin. Latex beads coated with microvillar membrane disks move in a myosin-like fashion along Nitella actin cables at rates of 12-60 nm/s (average rate of 33 nm/s); unlike purified BB myosin I, the movement of membrane disk-coated beads was most reproducibly observed in buffers containing low Ca2+.

Binding of myosin I to membrane lipids

The single-headed myosins called myosin-I were first isolated from the protozoan Acanthamoeba and subsequently identified in other cells. We previously reported evidence that myosin-I is responsible for the movement of membranes, extracted from Acanthamoeba, along actin filaments in vitro. Here we show for the first time that myosin-I can bind directly to NaOH-extracted membranes isolated from Acanthamoeba and to vesicles of pure lipids with an affinity sufficient for extensive binding in the cell. Membrane-bound myosin-I may provide a mechanism for many cellular movements previously thought to involve filamentous myosin-II.

Role of microtubules in polarized delivery of apical membrane proteins to the brush border of the intestinal epithelium

Colchicine- and vinblastine-induced depolymerization of microtubules (MTs) in the intestinal epithelium of rats and mice resulted in significant delivery of three apical membrane proteins (alkaline phosphatase, sucrase-isomaltase, and aminopeptidase N) to the basolateral membrane domain. In addition, typical brush borders (BBs) occurred at the basolateral cell surface, consisting of numerous microvilli that contained the four major components of the cytoskeleton of apical microvilli (actin, villin, fimbrin, and the 110-kD protein). Formation of basolateral microvilli required polymerization of actin and proceeded at glycocalyx-studded plaques that resembled the dense plaques located at the tips of apical microvilli. BBs from the basolateral membrane became internalized into BB-containing vacuoles which served as recipient organelles for newly synthesized apical membrane proteins. The BB vacuoles fused with each other and finally were inserted into the apical BB. Polarized distribution of Na+,K+-ATPase, a basolateral membrane protein, was not affected by drug-induced depolymerization of MTs. These observations indicate that Golgi-derived carrier vesicles (CVs) containing apical membrane proteins are vectorially guided to the apical cell surface by a retrograde transport along MTs. MTs are uniformly oriented towards a narrow space underneath the apical terminal web (termed subterminal space) that contains MT-organizing properties and controls polarized alignment of MTs. In contrast to apical CVs, targeting of basolateral CVs appears to be independent of MTs but demands a barrier at the apical membrane domain that prevents basolateral CVs from apical fusion (transport barrier hypothesis).

The 110-kD protein-calmodulin complex of the intestinal microvillus (brush border myosin I) is a mechanoenzyme

The 110-kD protein-calmodulin complex (110K-CM) of the intestinal brush border serves to laterally tether microvillar actin filaments to the plasma membrane. Results from several laboratories have demonstrated that this complex shares many enzymatic and structural properties with myosin. The mechanochemical potential of purified avian 110K-CM was assessed using the Nitella bead motility assay (Sheetz, M. P., and J. A. Spudich. 1983. Nature (Lond.). 303:31-35). Under low Ca2+ conditions, 110K-CM-coated beads bound to actin cables, but no movement was observed. Using EGTA/calcium buffers (approximately 5-10 microM free Ca2+) movement of 110K-CM-coated beads along actin cables (average rate of approximately 8 nm/s) was observed. The movement was in the same direction as that for beads coated with skeletal muscle myosin. The motile preparations of 110K-CM were shown to be free of detectable contamination by conventional brush border myosin. Based on these and other observations demonstrating the myosin-like properties of 110K-CM, we propose that this complex be named "brush border myosin I."