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Myosin-1a is one of eight monomeric, membrane binding class I myosins expressed in vertebrates.1 As the most abundant actin-based motor protein found in the enterocyte microvillus, myosin-1a has long been known to interact with the apical membrane via a highly basic C-terminal tail domain.2 Several recent studies shed light on possible functional consequences of this protein/lipid interaction. In vitro and in vivo studies of microvillar function have revealed that myosin-1a can move apical membrane along core actin bundles, leading to the release of small vesicles from microvillar tips.3,4 Additional studies indicate that myosin-1a and other class I myosins contribute to membrane-cytoskeleton adhesion, which enables the apical membrane to resist deformation.5 These findings clearly position myosin-1a as an important player in apical membrane movement and structural stability. How this motor is able to fulfill these two seemingly distinct functions is currently unclear, but will serve as the focus of our discussion below.
The intestinal epithelial cell brush border is one of the most highly organized F-actin arrays observed in biology. Brush borders consist of thousands of slender, finger-like protrusions of apical membrane known as microvilli, which are supported by parallel bundles of F-actin. Microvilli extend from the cell surface to an identical length and exhibit near perfect hexagonal packing.6 Not surprisingly, this actin-rich assemblage is composed of a wide variety of actin-binding proteins, many of which function in controlling the dynamics of F-actin assembly/disassembly; they also serve to modulate the mechanical properties and dimensions of the supporting actin bundle.7 In addition, this domain is home to a variety of actin-based motor proteins known as “myosins”. While our understanding of myosin function in this biological setting has been developing for many years, the latest studies have highlighted some unexpected and unconventional applications for myosin mechanical activity within the microvillus. These investigations have focused on the class I monomeric membrane-binding motor known as myosin-1a (Myo1a); an abundant microvillar component that links the overlying plasma membrane to the underlying actin bundle. Recent cell biological and biophysical studies have highlighted novel roles for this motor in the unique cytoskeletal context provided by the brush border.
Two recent investigations suggest that microvillar Myo1a is mechanically active and can power the plus-end directed (tipward) sliding of apical membrane along core actin bundles.3,4 In the context of intact, native enterocytes, this activity may be linked to the production and shedding of small vesicles from the tips of microvilli. Biochemical analysis of these vesicles has revealed that one of the most prominent protein cargoes is intestinal alkaline phosphatase, a brush border enzyme that was recently recognized as a gut host defense factor.8,9 The precise mechanism underlying microvillar membrane shedding has not been elucidated, but Myo1a is most likely generating plus-end directed forces that lead to the tip-ward movement of specific vesicle components and accumulation of apical membrane at microvillar tips.
Other recent experiments have focused on a seemingly distinct aspect of myosin- I function in the microvillus: the control of apical membrane tension.5 Pioneering studies on cell membrane mechanics established that physical bonding with the actin cytoskeleton accounts for the majority of tension (i.e., in plane stiffness) observed in cellular plasma membranes.10 For example, regions of the cell membrane that lose contact with the underlying actin cytoskeleton are prone to form spherical membrane protrusions known as “blebs”.11 In the complex case provided by the brush border, the amount of apical membrane supported by protruding microvillar actin bundles is ~100-fold greater than a simple flat surface would allow. Stabilizing this massive excess of membrane requires high levels of membrane-cytoskeleton (M-C) adhesion energy. Because Myo1a has the potential to serve as a bivalent linkage, simultaneously binding to the inner leaflet of the apical membrane and the core actin bundle, this motor is well suited to contribute to M-C adhesion. Indeed, bleb-like herniations were observed on the apical surface of enterocytes from Myo1a KO mouse.12 Direct physical measurements supporting this hypothesis were provided in recent studies that employed optical trapping to directly measure membrane tension in brush borders isolated from Myo1a KO mice or in cultured epithelial cells where the complement of Myo1a was disrupted.5 The results clearly show that Myo1a and other class I myosins play an important role in enabling the membrane to closely follow the contours of the underlying actin cytoskeleton and resist deformation.
Together, these recent investigations suggest that Myo1a has the potential to function in two distinct processes: (1) the movement of apical membrane along the core actin bundle, and (2) the adhesion of apical membrane to the core actin bundle. These findings leave us with a physical conundrum, as directed movement would require Myo1a motors to frequently detach from the actin core during their ATPase cycle. Obviously, detached molecules would be unable to contribute to M-C adhesion, and thus membrane tension. How does the ensemble of Myo1a within the microvillus perform these two seemingly disparate functions? We briefly consider a few possibilities below:
The three possibilities discussed here represent a subset of models that could explain how Myo1a might carry out multiple distinct functions in a common subcellular structure. Studies exploiting high resolution live cell imaging coupled with new probes that report on the conformation and dynamics of this motor are required before we achieve a satisfactory understanding of the mechanism underlying Myo1a multifunctionality.
This work was supported by grants from the National Institutes of Health (R01 DK-075555, M.J.T.) and the American Heart Association (09GRNT2310188, M.J.T.; Post-doctoral Fellowship 0825358E, R.N.).
Previously published online: www.landesbioscience.com/journals/cib/article/10141