Phospholipid membrane-associated brush border myosin-I activity

Myosins and membrane trafficking in intestinal brush border assembly

Myosins are a class of motors that participate in a wide variety of cellular functions including organelle transport, cell adhesion, endocytosis and exocytosis, movement of RNA, and cell motility. Among the emerging roles for myosins is regulation of the assembly, morphology, and function of actin protrusions such as microvilli. The intestine harbors an elaborate apical membrane composed of highly organized microvilli. Microvilli assembly and function are intricately tied to several myosins including Myosin 1a, non-muscle Myosin 2c, Myosin 5b, Myosin 6, and Myosin 7b. Here, we review the research progress made in our understanding of myosin mediated apical assembly.

Overexpression and purification of human myosins from transiently and stably transfected suspension adapted HEK293SF-3F6 cells

The myosin family of motor proteins is an attractive target of therapeutic small-molecule protein inhibitors and modulators. Milligrams of protein quantities are required to conduct proper biophysical and biochemical studies to understand myosin functions. Myosin protein expression and purification represent a critical starting point towards this goal. Established utilization of Dictyostelium discoideum, Drosophila melanogaster, insect and mouse cells for myosin expression and purification is limited, cost, labor and time inefficient particularly for (full-length) human myosins. Here we are presenting detailed protocols for production of several difficult-to-purify recombinant human myosins in efficient quantities up to 1 mg of protein per liter of cell culture. This is the first time that myosins have been purified in large scales from suspension adapted transiently and stably expressing human cells. The method is also useful for expressing other human proteins in quantities sufficient to perform extensive biochemical and biophysical characterization.

Membrane-bound myo1c powers asymmetric motility of actin filaments

Class I myosins are molecular motors that link cellular membranes to the actin cytoskeleton and play roles in membrane tension generation, membrane dynamics, and mechanosignal transduction. The widely expressed myosin-Ic (myo1c) isoform binds tightly to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] via a pleckstrin homology domain located in the myo1c tail, which is important for its proper cellular localization. In this study, we found that myo1c can power actin motility on fluid membranes composed of physiological concentrations of PtdIns(4,5)P(2) and that this motility is inhibited by high concentrations of anionic phospholipids. Strikingly, this motility occurs along curved paths in a counterclockwise direction (i.e., the actin filaments turn in leftward circles). A biotinylated myo1c construct containing only the motor domain and the lever arm anchored via streptavidin on a membrane containing biotinylated lipid can also generate asymmetric motility, suggesting that the tail domain is not required for the counterclockwise turning. We found that the ability to produce counterclockwise motility is not a universal characteristic of myosin-I motors, as membrane-bound myosin-Ia (myo1a) and myosin-Ib (myo1b) are able to power actin gliding, but the actin gliding has no substantial turning bias. This work reveals a possible mechanism for establishing asymmetry in relationship to the plasma membrane.

Myo1e binds anionic phospholipids with high affinity

Myo1e is a single-headed motor protein that has been shown to play roles in clathrin-mediated endocytosis in HeLa cells and podocyte function in the kidney. The myo1e C-terminal tail domain includes a basic region that is required for localization to clathrin-coated vesicles and contains a putative pleckstrin-homology (PH) domain that has been shown to play a role in phospholipid binding in other myosin-I proteins. We used sedimentation assays, stopped-flow fluorescence, and fluorescence microscopy to determine the membrane binding affinities, kinetics, and in vivo localization of fluorescently labeled recombinant myo1e-tail constructs. We found that the myo1e tail binds tightly to large unilamellar vesicles (LUVs) containing physiological concentrations of the anionic phospholipids phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) or phosphatidylserine. The rate of myo1e attachment to LUVs nears the diffusion limit while the calculated rate of detachment from LUVs is slow (k(diss) ≤ 0.4 s(-1)). Mutation of conserved residues in the myo1e PH domain has little effect on lipid binding in vitro or membrane localization in vivo. Soluble inositol phosphate headgroups, such as inositol 1,4,5-trisphosphate, can compete with PtdIns(4,5)P(2) for binding, but the apparent affinity for the soluble inositol phosphate is substantially lower than that for PtdIns(4,5)P(2). These results suggest that myo1e binds lipids through nonspecific electrostatic interactions rather than a stereospecific protein-phosphoinositide interaction.

Myosin-1a powers the sliding of apical membrane along microvillar actin bundles

Microvilli are actin-rich membrane protrusions common to a variety of epithelial cell types. Within microvilli of the enterocyte brush border (BB), myosin-1a (Myo1a) forms an ordered ensemble of bridges that link the plasma membrane to the underlying polarized actin bundle. Despite decades of investigation, the function of this unique actomyosin array has remained unclear. Here, we show that addition of ATP to isolated BBs induces a plus end–directed translation of apical membrane along microvillar actin bundles. Upon reaching microvillar tips, membrane is “shed” into solution in the form of small vesicles. Because this movement demonstrates the polarity, velocity, and nucleotide dependence expected for a Myo1a-driven process, and BBs lacking Myo1a fail to undergo membrane translation, we conclude that Myo1a powers this novel form of motility. Thus, in addition to providing a means for amplifying apical surface area, we propose that microvilli function as actomyosin contractile arrays that power the release of BB membrane vesicles into the intestinal lumen.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Six putative IQ motifs of the recombinant chicken intestinal brush border myosin I are involved in calmodulin binding

Chicken brush border myosin I has up to six IQ sequence motifs at which it may bind calmodulin. To determine the relative contributions of these motifs to calmodulin binding, fusion deletion fragments were expressed in Escherichia coli and their ability to bind calmodulin was assessed by the gel overlay technique. The first three N-terminal IQ sites showed strong binding with calmodulin. Surprisingly, the last three incomplete IQ motifs also contributed substantial calmodulin binding. The first and fourth IQ sites bound calmodulin but tended to reduce binding in combination with other sites. The data indicate that interactions among all six IQ motifs contribute to the ability of myosin I to bind calmodulin.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Kinetic characterization of brush border myosin-I ATPase

Brush border myosin-I (BBM-I) is a single-headed unconventional myosin found in the microvilli of intestinal epithelial cells. We used stopped-flow kinetic analysis to measure the rate and equilibrium constants for several steps in the BBM-I ATPase cycle. We determined the rates for ATP binding to BBM-I and brush border actomyosin-I (actoBBM-I), the rate of actoBBM-I dissociation by ATP, and the rates for the steps in ADP dissociation from actoBBM-I. The rate and equilibrium constants for several of the steps in the actoBBM-I ATPase are significantly different from those of other members of the myosin superfamily. Most notably, dissociation of the actoBBM-I complex by ATP and release of ADP from actoBBM-I are both very slow. The slow rates of these steps may play a role in lengthening the time spent in force-generating states and in limiting the maximal rate of BBM-I motility. In addition, release of ADP from the actoBBM-I complex occurs in at least two steps. This study provides evidence for a member of the myosin superfamily with markedly divergent kinetic behavior.

Brush border myosin-I structure and ADP-dependent conformational changes revealed by cryoelectron microscopy and image analysis

Brush border myosin-I (BBM-I) is a single-headed myosin found in the microvilli of intestinal epithelial cells, where it forms lateral bridges connecting the core bundle of actin filaments to the plasma membrane. Extending previous observations (Jontes, J.D., E.M. Wilson-Kubalek, and R.A. Milligan. 1995. Nature [Lond.]. 378:751-753), we have used cryoelectron microscopy and helical image analysis to generate three-dimensional (3D) maps of actin filaments decorated with BBM-I in both the presence and absence of 1 mM MgADP. In the improved 3D maps, we are able to see the entire light chain-binding domain, containing density for all three calmodulin light chains. This has enabled us to model a high resolution structure of BBM-I using the crystal structures of the chicken skeletal muscle myosin catalytic domain and essential light chain. Thus, we are able to directly measure the full magnitude of the ADP-dependent tail swing. The approximately 31 degrees swing corresponds to approximately 63 A at the end of the rigid light chain-binding domain. Comparison of the behavior of BBM-I with skeletal and smooth muscle subfragments-1 suggests that there are substantial differences in the structure and energetics of the biochemical transitions in the actomyosin ATPase cycle.

Zipper protein, a B-G protein with the ability to regulate actin/myosin 1 interactions in the intestinal brush border

We recently identified a 28-kDa protein in the intestinal brush border that resembled tropomyosin in terms of size, homology, and alpha helical content. This protein contained 27 heptad repeats, nearly all of which began with leucine, leading to its name zipper protein. Subsequent analysis, however, indicated that both a 49-kDa and a 28-kDa immunoreactive protein existed in intestinal brush-border extracts. Using 5'-rapid amplification of cDNA ends analysis, we extended the N-terminal sequence of zipper protein to the apparent translation start site. This additional sequence contained a putative transmembrane domain and two potential tryptic cleavage sites C-terminal to the transmembrane domain which would release a 28-kDa cytoplasmic protein if utilized. The additional sequence was highly homologous to members of the B-G protein family, a family with no known function. Immunoelectron microscopy showed that zipper protein was confined to the membrane of the microvillus where it was in close association with brush-border myosin 1 (BBM1). Recombinant zipper protein (28-kDa cytoplasmic portion) blocked the binding of actin to BBM1 and inhibited actin-stimulated BBM1 ATPase activity. In contrast, zipper protein had no effect on endogenous or K/EDTA-stimulated BBM1 ATPase activity. Furthermore, zipper protein displaced tropomyosin from binding to actin, suggesting that these homologous proteins bind to the same sites on the actin molecule. We conclude that zipper protein is a transmembrane protein of the B-G family localized to the intestinal epithelial cell microvillus. The extended cytoplasmic tail either in the intact molecule or after tryptic cleavage may participate in regulating the binding and, thus, activation of BBM1 by actin in a manner similar to tropomyosin.

Mammalian myosin I alpha is concentrated near the plasma membrane in nerve growth cones

To determine if unconventional myosins play a role in nerve outgrowth, antibodies specific for rat brain derived mammalian myosin I alpha (MMI alpha) were used to label cultured rat superior cervical ganglion nerve cells. Observations were made at both the light and electron microscopic level of resolution using preparative procedures designed to enhance the ability to precisely determine the relationship between antibody label and cellular structures in order to map the distribution and structural association of this myosin. Immunofluorescence showed that MMI alpha has a punctate distribution throughout the nerve cell body, neurites, and growth cones. In growth cones, MMI alpha staining is sometimes elevated in thin peripheral regions of high actin content at the leading edge. Immunoelectron microscopy using colloidal gold conjugated antibodies showed that in growth cones MMI alpha is absent from membranous organelles and is concentrated primarily in the cell cortex adjacent to the cell membrane. The cortical label is equally distributed between upper and lower membranes. The plasma membrane association of the MMI alpha label persists under conditions in which the actin cytoskeleton is perturbed or removed, suggesting a direct association between a fraction of MMI alpha and the plasma membrane. MMI alpha label is also associated with the non-cortical actin cytoskeleton. Partial disruption of the actin cytoskeleton using cytochalasin B causes redistribution of only a subset of MMI alpha label. These data suggest a complex relationship between MMI alpha, the actin cytoskeleton, and the plasma membrane in the growth cone. In contrast to its localization in the growth cone, in neuronal cell bodies MMI alpha is also associated with tubulovesicular structures. This suggests that at this location MMI alpha may either act as an organelle motor or is passively transported to the plasma membrane on vesicles.

Regulation of calmodulin-binding myosins

Myosins constitute a diverse superfamily of actin-based mechanoenzymes that are involved in many essential cellular motilities. In addition to conventional muscle myosin II, ten other classes of unconventional myosins are known. Many unconventional myosins bind multiple calmodulin light chains and Ca2+, which can dramatically alter their mechanochemical and enzymatic activity. Calmodulin-binding myosins can also be regulated by phospholipid binding, phosphorylation of the heavy chain and actin-binding proteins. The molecular details linking unconventional-myosin regulation and function are just beginning to emerge.