Chapter 3 Structural and Functional Dissection of a Membrane-Bound Mechanoenzyme: Brush Border Myosin I

Abstract

The only vertebrate myosin I that has been thoroughly characterized is termed brush border (BB) myosin I and is expressed within the microvilli of the intestinal epithelial cell. This chapter discusses the structural and functional properties of brush border myosin I— the subunit composition, the protein domain structure, and the primary structure of its heavy chain— with compliment of multiple calmodulin (CM) light chains. The chapter also discusses the acid binding properties— that is, stoichiometry and Ca2+ dependence and actin filament cross-linking by brush border Myosin I. It describes the adenosine triphosphate (ATP)ase properties of the brush border myosin I, its mechanochemistry, the function of brush border myosin I calmodulin light chains, the role of calmodulin light chains as repressors rather than activators of brush border myosin I Mg2+-ATPase. The chapter discusses the interaction of brush border myosin I, with the microvillar membrane, with acidic phospholipids, the evidence for a microvillar membrane “Docking” protein for brush border myosin I and notions regarding brush border myosin I function. A ∼140-kDa glycoprotein (GP-140) binds to the heavy chain of BB myosin I. GP-140 is the linker protein that tethers BB myosin I to the microvillus (MV) membrane. The disk-bound BB myosin I molecules are tethered to the membrane by their tails. The bound BB myosin I retain all the activities, associated with the free molecule, including actin binding and mechanochemical activity. BB myosin I might participate in the biogenesis and recycling of the apical membrane, by transporting vesicles, up through the terminal web. BB myosin I could cause a relative downward or perhaps rotational movement of the core within the MV membrane and could promote bulk movement of the membrane upward along the MV axis, as a part of an active vesicular shedding mechanism.