The 110-kD protein-calmodulin complex of the intestinal microvillus is an actin-activated MgATPase.

Abstract

Abstract. The microvillus ll0-kD protein-calmodulin complex (designated ll0K-CM) shares several properties with all myosins. In addition to its well-defined ATP-dependent binding interaction with F-actin, ll0K-CM is an ATPase with diagnostically myosin-like divalent cation sensitivity. It exhibits maximum enzymatic activity in the presence of K + and EDTA (0.24 ~tmol Pi/mg per min) or in the presence of Ca ÷+ (0.40 ~tmol P~/mg per min) and significantly less activity in physiological ionic conditions of salt and Mg++ (0.04 ~tmol Pi/mg per min). This MgATPase is activated by F-actin in an actin concentration-dependent manner (up to 2.5-3.5-fold). The specific MgATPase activity of ll0K-CM is also enhanced by the addition of 5-10 ~tM Ca +÷, but in the isolated complex, there is often also a decrease in the extent of actin activation in this range of free Ca +÷. Actin activation is maintained, however, in samples with exogenously added calmodulin; under these conditions, there is an approximately sevenfold stimulation of ll0K-CM's enzymatic activity in the presence of 5-10 lxM Ca ++ and actin, ll0K-CM is relatively indiscriminant in its nucleoside triphosphate specificity; in addition to ATP, GTP, CTP, UTP, and ITP are all hydrolyzed by the complex in the presence of either Mg++ or Ca ++. Neither AMP nor the phosphatase substrate p-nitrophenyl phosphate are substrates for the enzymatic activity. The pH optimum for CaATPase activity is 6.0-7.5; maximum actin activation of MgATPase occurs over a broad pH range of 6.5-8.5. Finally, like myosins, purified ll0K-CM crosslinks actin filaments into loosely ordered aggregates in the absence of ATP. Collectively these data support the proposal of Collins and Borysenko (1984, J. Biol. Chem., 259:14128-14135) that the ll0K-CM complex is functionally analogous to the mechanoenzyme myosin.

Interaction of Arabidopsis kinesin‐like calmodulin‐binding protein with tubulin subunits: modulation by Ca2+‐calmodulin

“…In the case of brush border myosin I, calcium causes partial dissociation of calmodulin from the motor resulting in complete inhibition of motility (Wolenski et al, 1993). Also, calcium has profound effects on actin-independent and actin-dependent MgATPase activity (Collins et al, 1990;Cozzelman and Mooseker, 1987;Swanljung-Collins and Collins, 1991;Wolenski et al, 1993). The observation that the amino terminal protein sequence of the three KCBPs that we have isolated show homology to myosins (Chen et al, 1996;Reddy et al, 1996b) suggests that KCBP contains features of both microtubule-and actin-based motors.…”

Organization of the actin filament cytoskeleton in the intestinal brush border: a quantitative and qualitative immunoelectron microscope study.

“…Thus, villin can no longer bundle the actin filaments (10) and can no longer cause depolymerization of the filaments at elevated levels of Ca ++ (1,10,54,71). Given that the bundling activity of fimbrin is not inhibited by tropomyosin, and considering that the 110-kD protein causes lateral association between actin filaments in vitro (13,14), the present demonstration of fimbrin and ll0-kD protein in the rootlets may help to explain why rootlet filaments are still in a bundled state. Surprisingly, IgG-gold label specific for villin and fimbrin was •20% less concentrated in the rootlets than in the microvillus core bundle although the density of anti-actin label was the same in both the rootlets and microvilli.…”

“…Two recent studies indicate that the 110-kD protein shares several properties of a myosin-like mechanoenzyme (12,14). Its ability to bind with one end to actin filaments and with the other to components of the lipid bilayer has led to the suggestion that this Figure 9.…”

“…Thus, the rootlets should contain further cross-linking proteins that maintain the actin filaments bundled. With respect to the ll0-kD protein, there is increasing evidence that it shares several properties of a myosin-related mechanoprotein (12,14). This has led to the suggestion that the ll0-kD protein may not only link the core bundle to the plasma membrane (13,31,37,46,47) but may also connect vesicles to the rootlet bundles and thus may play a role in transport of vesicles through the terminal web (14).…”

A second isoform of chicken brush border myosin I contains a 29‐residue inserted sequence that binds calmodulin

“…This complex, which has been studied most extensively using chicken intestinal tissue as a source, shares many properties with myosins, including an actin-activated Mg"-ATPase activity [3,4] and mechanochemical activity [5,6]. The sequence of an authentic, near-fulllength cDNA clone for the heavy chain of chicken BB-MI reveals a -119 kDa polypeptide that is composed of an -82 kDa myosin globular head ('Sl') domain fused to a -37 kDa tail domain that bears no resemblance to the rod-like tail of conventional myosins [7] (see Fig.…”