Brush border myosin I has a calmodulin/phosphatidylserine switch and tail actin-binding

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.

An experimentally based computer search identifies unstructured membrane-binding sites in proteins: application to class I myosins, PAKS, and CARMIL

Programs exist for searching protein sequences for potential membrane-penetrating segments (hydrophobic regions) and for lipid-binding sites with highly defined tertiary structures, such as PH, FERM, C2, ENTH, and other domains. However, a rapidly growing number of membrane-associated proteins (including cytoskeletal proteins, kinases, GTP-binding proteins, and their effectors) bind lipids through less structured regions. Here, we describe the development and testing of a simple computer search program that identifies unstructured potential membrane-binding sites. Initially, we found that both basic and hydrophobic amino acids, irrespective of sequence, contribute to the binding to acidic phospholipid vesicles of synthetic peptides that correspond to the putative membrane-binding domains of Acanthamoeba class I myosins. Based on these results, we modified a hydrophobicity scale giving Arg- and Lys-positive, rather than negative, values. Using this basic and hydrophobic scale with a standard search algorithm, we successfully identified previously determined unstructured membrane-binding sites in all 16 proteins tested. Importantly, basic and hydrophobic searches identified previously unknown potential membrane-binding sites in class I myosins, PAKs and CARMIL (capping protein, Arp2/3, myosin I linker; a membrane-associated cytoskeletal scaffold protein), and synthetic peptides and protein domains containing these newly identified sites bound to acidic phospholipids in vitro.

Kinetics of the interaction of myo1c with phosphoinositides

myo1c is a single-headed myosin that dynamically links membranes to the actin cytoskeleton. A putative pleckstrin homology domain has been identified in the myo1c tail that binds phosphoinositides and soluble inositol phosphates with high affinity. However, the kinetics of association and dissociation and the influence of phospholipid composition on the kinetics have not been determined. Stopped-flow spectroscopy was used to measure the binding and dissociation of a recombinant myo1c construct containing the tail and regulatory domains (myo1c(IQ-tail)) to and from 100-nm diameter large unilamellar vesicles (LUVs). We found the time course of association of myo1c(IQ-tail) with LUVs containing 2% phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) followed a two-exponential time course, and the rate of the predominant fast phase depended linearly upon the total lipid concentration. The apparent second-order rate constant was approximately diffusion-limited. Increasing the molar ratio of anionic phospholipid by adding phosphatidylserine, additional PtdIns(4,5)P(2), or by situating PtdIns(4,5)P(2) in a more physiologically relevant lipid background increased the apparent association rate constant less than 2-fold. myo1c(IQ-tail) dissociated from PtdIns(4,5)P(2) at a slower rate (2.0 s(-1)) than the pleckstrin homology domain of phospholipase C-delta (13 s(-1)). The presence of additional anionic phospholipid reduced the myo1c(IQ-tail) dissociation rate constant >50-fold but marginally changed the dissociation rate of phospholipase C-delta, suggesting that additional electrostatic interactions in myo1c(IQ-tail) help to stabilize binding. Remarkably, high concentrations of soluble inositol phosphates induce dissociation of myo1c(IQ-tail) from LUVs, suggesting that phosphoinositides are able to bind to and dissociate from myo1c(IQ-tail) as it remains bound to the membrane.

Expression of the unconventional myosin Myo1c alters sodium transport in M1 collecting duct cells

Epithelial cells rely on proper targeting of cellular components to perform their physiological function. This dynamic process utilizes the cytoskeleton and involves movement of vesicles to and from the plasma membrane, thus traversing the actin cortical cytoskeleton. Studies support both direct interaction of actin with channels and an indirect mechanism whereby actin may serve as a track in the final delivery of the channel to the plasma membrane. Actin-dependent processes are often mediated via a member of the myosin family of proteins. Myosin I family members have been implicated in multiple cellular events occurring at the plasma membrane. In these studies, we investigated the function of the unconventional myosin I Myo1c in the M1 mouse collecting duct cell line. Myo1c was observed to be concentrated at or near the plasma membrane, often in discrete membrane domains. To address the possible role of Myo1c in channel regulation, we expressed a truncated Myo1c, lacking ATP and actin domains, in M1 cells and compared electrophysiological responses to control M1 cells, M1 cells expressing the empty vector, and M1 cells expressing the full-length Myo1c construct. Interestingly, cells expressing the Myo1c constructs had modulated antidiuretic hormone (ADH)-stimulated short-circuit current and showed little inhibition of short-circuit current with amiloride addition. Evaluation of enhanced green fluorescent protein-Myo1c constructs supports the importance of the IQ region in targeting the Myo1c to its respective cellular domain. These data are consistent with Myo1c participating in the regulation of the Na+ channel after ADH stimulation.

Trk is a calmodulin-binding protein: implications for receptor processing

The tyrosine kinase receptors for the neurotrophins (Trk) are a family of transmembrane receptors that regulate the differentiation and survival of different neuronal populations. Neurotrophin binding to Trk leads to the activation of several signalling pathways including a rapid, but moderate, increase in intracellular calcium levels. We have previously described the role of calcium and its sensor protein, calmodulin, in Trk-activated intracellular pathways. Here we demonstrate that calmodulin is able to precipitate TrkA from PC12 cell lysates. Using recombinant GST-fusion proteins containing the complete intracellular domain of TrkA, or fragments of this region, we show that calmodulin binds directly to the C-terminal domain of TrkA in a Ca2+-dependent manner. We have also co-immunoprecipitated endogenous Trk and calmodulin in primary cultures of cortical neurones. Moreover, we provide evidence that calmodulin is involved in the regulation of TrkA processing in PC12 cells. Calmodulin inhibition results in the generation of a TrkA-derived p41 fragment from the cytosolic portion of the protein. This fragment is autophosphorylated in tyrosines and can recruit PLCgamma and Shc adaptor proteins. These results suggest that calmodulin binding to Trk may be important for the regulation of Trk intracellular localization and cleavage.

DREAM-alphaCREM interaction via leucine-charged domains derepresses downstream regulatory element-dependent transcription

Protein kinase A-dependent derepression of the human prodynorphin gene is regulated by the differential occupancy of the Dyn downstream regulatory element (DRE) site. Here, we show that a direct protein-protein interaction between DREAM and the CREM repressor isoform, alphaCREM, prevents binding of DREAM to the DRE and suggests a mechanism for cyclic AMP-dependent derepression of the prodynorphin gene in human neuroblastoma cells. Phosphorylation in the kinase-inducible domain of alphaCREM is not required for the interaction, but phospho-alphaCREM shows higher affinity for DREAM. The interaction with alphaCREM is independent of the Ca(2+)-binding properties of DREAM and is governed by leucine-charged residue-rich domains located in both alphaCREM and DREAM. Thus, our results propose a new mechanism for DREAM-mediated derepression that can operate independently of changes in nuclear Ca(2+).

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.

Calcium-induced conformational transition revealed by the solution structure of apo calmodulin

The solution structure of Ca(2+)-free calmodulin has been determined by NMR spectroscopy, and is compared to the previously reported structure of the Ca(2+)-saturated form. The removal of Ca2+ causes the interhelical angles of four EF-hand motifs to increase by 36 degrees-44 degrees. This leads to major changes in surface properties, including the closure of the deep hydrophobic cavity essential for target protein recognition. Concerted movements of helices A and D with respect to B and C, and of helices E and H with respect to F and G are likely responsible for the cooperative Ca(2+)-binding property observed between two adjacent EF-hand sites in the amino- and carboxy-terminal domains.