Partial deduced sequence of the 110-kD-calmodulin complex of the avian intestinal microvillus shows that this mechanoenzyme is a member of the myosin I family

Discovery of the first unconventional myosin: Acanthamoeba myosin-I

Having characterized actin from Acanthamoeba castellanii (Weihing and Korn, Biochemistry, 1971, 10, 590-600) and knowing that myosin had been isolated from the slime mold Physarum (Hatano and Tazawa, Biochim. Biophys. Acta, 1968, 154, 507-519; Adelman and Taylor, Biochemistry, 1969, 8, 4976-4988), we set out in 1969 to find myosin in Acanthamoeba. We used K-EDTA-ATPase activity to assay myosin, because it is a unique feature of muscle myosins. After slightly less than 3 years, we purified a K-EDTA ATPase that interacted with actin. Actin filaments stimulated the Mg-ATPase activity of the crude enzyme, but this was lost with further purification. Recombining fractions from the column where this activity was lost revealed a "cofactor" that allowed actin filaments to stimulate the Mg-ATPase of the purified enzyme. The small size of the heavy chain and physical properties of the purified myosin were unprecedented, so many were skeptical, assuming that our myosin was a proteolytic fragment of a larger myosin similar to muscle or Physarum myosin. Subsequently our laboratories confirmed that Acanthamoeba myosin-I is a novel unconventional myosin that interacts with membrane lipids (Adams and Pollard, Nature, 1989, 340 (6234), 565-568) and that the cofactor is a myosin heavy chain kinase (Maruta and Korn, J. Biol. Chem., 1977, 252, 8329-8332). Phylogenetic analysis (Odronitz and Kollmar, Genome Biology, 2007, 8, R196) later established that class I myosin was the first myosin to appear during the evolution of eukaryotes.

Non-muscle myosins in tumor progression, cancer cell invasion, and metastasis

The actin cytoskeleton, which regulates cell polarity, adhesion, and migration, can influence cancer progression, including initial acquisition of malignant properties by normal cells, invasion of adjacent tissues, and metastasis to distant sites. Actin-dependent molecular motors, myosins, play key roles in regulating tumor progression and metastasis. In this review, we examine how non-muscle myosins regulate neoplastic transformation and cancer cell migration and invasion. Members of the myosin superfamily can act as either enhancers or suppressors of tumor progression. This review summarizes the current state of knowledge on how mutations or epigenetic changes in myosin genes and changes in myosin expression may affect tumor progression and patient outcomes and discusses the proposed mechanisms linking myosin inactivation or upregulation to malignant phenotype, cancer cell migration, and metastasis.

A two-photon FRAP analysis of the cytoskeleton dynamics in the microvilli of intestinal cells

The molecular structure of the brush-border of enterocytes has been investigated since the 1980s, but the dynamics of this highly specialized subcellular domain have been difficult to study due to its small size. To perform a detailed analysis of the dynamics of cytoskeleton proteins in this domain, we developed two-photon fluorescence recovery after photobleaching and a theoretical framework for data analysis. With this method, fast dynamics of proteins in the microvilli of the brush border of epithelial intestinal cells can be measured on the millisecond timescale in volumes smaller than 1 microm3. Two major proteins of the cytoskeleton of the microvilli, actin and myosin 1a (Myo1a; formerly named brush border myosin I), are mobile in the brush-border of Caco-2 cells, an enterocyte-like cellular model. However, the mobility of actin is very different from that of Myo1a and they appear to be unrelated (diffusion coefficient of 15 microm2 s(-1) with a mobile fraction of 60% for actin, and 4 microm2 s(-1) with a mobile fraction of 90% for Myo1a). Furthermore, we show for the first time, in vivo, that the dynamics of Myo1a in microvilli reflect its motor activity.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

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.

Truncation of a mammalian myosin I results in loss of Ca2+-sensitive motility

MYR-1, a mammalian class I myosin, consisting of a heavy chain and 4-6 associated calmodulins, is represented by the 130-kDa myosin I (or MI(130)) from rat liver. MI(130) translocates actin filaments in vitro in a Ca(2+)-regulated manner. A decrease in motility observed at higher Ca(2+) concentrations has been attributed to calmodulin dissociation. To investigate mammalian myosin I regulation, we have coexpressed in baculovirus calmodulin and an epitope-tagged 85-kDa fragment representing the amino-terminal catalytic "motor" domain and the first calmodulin-binding IQ domain of rat myr-1; we refer to this truncated molecule here as MI(1IQ). Association of calmodulin to MI(1IQ) is Ca(2+)-insensitive. MI(1IQ) translocates actin filaments in vitro at a rate resembling MI(130), but unlike MI(130), does not exhibit sensitivity to 0.1-100 micrometer Ca(2+). In addition to demonstrating successful expression of a functional truncated mammalian myosin I in vitro, these results indicate that: 1) Ca(2+)-induced calmodulin dissociation from MI(130) in the presence of actin is not from the first IQ domain, 2) velocity is not affected by the length of the IQ region, and 3) the Ca(2+) sensitivity of actin translocation exhibited by MI(130) involves 1 or more of the other 5 IQ domains and/or the carboxyl tail.

Human brush border myosin-I and myosin-Ic expression in human intestine and Caco-2BBe cells

The human intestinal cell line, Caco-2BBe, has been established as an excellent model system for analysis of the enterocyte cytoskeleton including that of the actin rich apical brush border. To facilitate its use for functional analysis of a major component of the brush border, brush border myosin-I, human cDNAs encoding the heavy chain of this class I myosin were isolated and sequenced. The identity of this myosin as human brush border myosin-I was verified based on similarity with other vertebrate sequences, as well as its expression profile at both the RNA and protein levels. Localization of the protein in human intestine along the crypt-villus axis closely resembles that previously determined for brush border myosin-I in chicken, and is quite distinct from that of myosin-Ic, another myosin-I expressed in human intestine and Caco-2BBe cells. In immature cells of the crypt, brush border myosin-I staining is low, and there is significant cytosolic and basolateral localization, while villus cells stain much more intensely, and the protein is primarily localized to the brush border. Localization of myosin-Ic is essentially the inverse of brush border myosin-I in that crypt cells exhibit higher levels of staining, while villus cells have very low levels of myosin-Ic. The expression of both myosins-I was also examined during cell-contact induced differentiation of Caco-2BBe cells where expression and changes in localization closely resemble those that accompany differentiation of enterocyte in vivo.

Truncated brush border myosin I affects membrane traffic in polarized epithelial cells

We investigate, in this study, the potential involvement of an acto-myosin-driven mechanism in endocytosis of polarized cells. We observed that depolymerization of actin filaments using latrunculin A decreases the rate of transferrin recycling to the basolateral plasma membrane of Caco-2 cells, and increases its delivery to the apical plasma membrane. To analyze whether a myosin was involved in endocytosis, we produced, in this polarized cell line, truncated, non-functional, brush border, myosin I proteins (BBMI) that we have previously demonstrated to have a dominant negative effect on endocytosis of unpolarized cells. These non-functional proteins affect the rate of transferrin recycling and the rate of transepithelial transport of dipeptidyl-peptidase IV from the basolateral plasma membrane to the apical plasma membrane. They modify the distribution of internalized endocytic tracers in apical multivesicular endosomes that are accessible to fluid phase tracers internalized from apical and basolateral plasma membrane domains. Altogether, these observations suggest that an acto-myosin-driven mechanism is involved in the trafficking of basolaterally internalized molecules to the apical plasma membrane.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

Apocalmodulin

Intracellular Ca2+ is normally maintained at submicromolar levels but increases during many forms of cellular stimulation. This increased Ca2+ binds to receptor proteins such as calmodulin (CaM) and alters the cell's metabolism and physiology. Calcium-CaM binds to target proteins and alters their function in such a way as to transduce the Ca2+ signal. Calcium-free or apocalmodulin (ApoCaM) binds to other proteins and has other specific effects. Apocalmodulin has roles in the cell that apparently do not require the ability to bind Ca2+ at all, and these roles appear to be essential for life. Apocalmodulin differs from Ca2+-CaM in its tertiary structure. It binds target proteins differently, utilizing different binding motifs such as the IQ motif and noncontiguous binding sites. Other kinds of binding potentially await discovery. The ApoCaM-binding proteins are a diverse group of at least 15 proteins including enzymes, actin-binding proteins, as well as cytoskeletal and other membrane proteins, including receptors and ion channels. Much of the cellular CaM is bound in a Ca2+-independent manner to membrane structures within the cell, and the proportion bound changes with cell growth and density, suggesting it may be a storage form. Apocalmodulin remains tightly bound to other proteins as subunits and probably hastens the response of these proteins to Ca2+. The overall picture that emerges is that CaM cycles between its Ca2+-bound and Ca2+-free states and in each state binds to different proteins and performs essential functions. Although much of the research focus has been on the roles of Ca2+-CaM, the roles of ApoCaM are equally vital but less well understood.

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.

Structural invariance of constitutively active and inactive mutants of acanthamoeba myosin IC bound to F-actin in the rigor and ADP-bound states

The three single-headed monomeric myosin I isozymes of Acanthamoeba castellanii (AMIs)-AMIA, AMIB, and AMIC-are among the best-studied of all myosins. We have used AMIC to study structural correlates of myosin's actin-activated ATPase. This activity is normally controlled by phosphorylation of Ser-329, but AMIC may be switched into constitutively active or inactive states by substituting this residue with Glu or Ala, respectively. To determine whether activation status is reflected in structural differences in the mode of attachment of myosin to actin, these mutant myosins were bound to actin filaments in the absence of nucleotide (rigor state) and visualized at 24-A resolution by using cryoelectron microscopy and image reconstruction. No such difference was observed. Consequently, we suggest that regulation may be affected not by altering the static (time-averaged) structure of AMIC but by modulating its dynamic properties, i.e., molecular breathing. The tail domain of vertebrate intestinal brush-border myosin I has been observed to swing through 31 degrees on binding of ADP. However, it was predicted on grounds of differing kinetics that any such effects with AMIC should be small [Jontes, J. D., Ostap, E. M., Pollard, T. D. & Milligan, R. A. (1998) J. Cell Biol. 141, 155-162]. We have confirmed this hypothesis by observing actin-associated AMIC in its ADP-bound state. Finally, we compared AMIC to brush-border myosin I and AMIB, which were previously studied under similar conditions. In each case, the shape and angle of attachment to F-actin of the catalytic domain is largely conserved, but the domain structure and disposition of the tail is distinctively different for each myosin.

Three-dimensional structure of Acanthamoeba castellanii myosin-IB (MIB) determined by cryoelectron microscopy of decorated actin filaments

The Acanthamoeba castellanii myosin-Is were the first unconventional myosins to be discovered, and the myosin-I class has since been found to be one of the more diverse and abundant classes of the myosin superfamily. We used two-dimensional (2D) crystallization on phospholipid monolayers and negative stain electron microscopy to calculate a projection map of a "classical" myosin-I, Acanthamoeba myosin-IB (MIB), at approximately 18 A resolution. Interpretation of the projection map suggests that the MIB molecules sit upright on the membrane. We also used cryoelectron microscopy and helical image analysis to determine the three-dimensional structure of actin filaments decorated with unphosphorylated (inactive) MIB. The catalytic domain is similar to that of other myosins, whereas the large carboxy-terminal tail domain differs greatly from brush border myosin-I (BBM-I), another member of the myosin-I class. These differences may be relevant to the distinct cellular functions of these two types of myosin-I. The catalytic domain of MIB also attaches to F-actin at a significantly different angle, approximately 10 degrees, than BBM-I. Finally, there is evidence that the tails of adjacent MIB molecules interact in both the 2D crystal and in the decorated actin filaments.

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.

Conformational changes due to calcium-induced calmodulin dissociation in brush border myosin I-decorated F-actin revealed by cryoelectron microscopy and image analysis

Brush border myosin I (BBMI) is a single-headed molecular motor. Its catalytic domain exhibits extensive sequence homology to the catalytic domain of myosin II, while its tail lacks the coiled-coil nature of myosin II. The BBMI tail domain contains at least three IQ motifs and binds calmodulin. Addition of calcium removes one of these calmodulin light chains, with effects on ATPase activity and motility in in vitro assays. Using the techniques of cryoelectron microscopy and helical image analysis we have calculated three-dimensional (3D) maps of BBMI-decorated actin filaments prepared in the presence and absence of calcium. The 3D maps describe a BBMI catalytic domain that is strikingly similar to the catalytic domain of myosin II subfragment 1 (S1), with the exception of a short amino-terminal region of the heavy chain, which is absent from BBMI. The tail domains of BBMI and S1 are highly divergent in structure, continuing on from their respective motor domains with very different geometries. Addition of calcium to BBMI, and the concomitant loss of a calmodulin light chain, results in an extensive reorganization of mass in the tail domain.

Three-dimensional structure of Brush Border Myosin-I at approximately 20 A resolution by electron microscopy and image analysis

Brush Border Myosin-I (BBMI) is a single-headed, unconventional myosin found in the microvilli of intestinal epithelial cells where it forms lateral bridges between the core bundle of actin filaments and the plasma membrane of the microvillus. A three-dimensional (3D) reconstruction of BBMI was made from images of negatively stained, two-dimensional (2D) crystals grown on lipid monolayers formed from mixtures of phosphatidylserine and phosphatidylcholine. The resolution of the 3D map extends to approximately 20 A and allows identification of all of the major structural domains of BBMI. The BBMI molecule is composed of three domains: a globular motor domain, a light-chain-binding domain and a lipid-binding domain. In our map, the putative motor domain is connected to an extended density, which we believe to be the light-chain-binding domain. This long, narrow region has three distinct bends, which may delineate the bound calmodulin light chains. Following the last calmodulin there is density which extends for a short distance across the lipid surface and is presumably the carboxy-terminal lipid-binding domain.

Chicken myosin IB mRNA is highly expressed in lymphoid tissues

Little is known about the functions of members of the myosin I family in vertebrates. Chicken myosin IB is a member of the amoeba-type subclass of myosin I molecules and tissue localisation studies may provide possible clues to the functions of these myosin I molecules. The expression of the mRNA of this unconventional myosin IB was analysed by in situ hybridization and compared with that of the well characterised brush border myosin I on frozen sections of tissues from the adult domestic chicken. High levels of myosin IB mRNA were found in the intestine and spleen, but were not found in other tissues examined such as brain, heart, lung, liver and kidney. In the intestine, myosin IB mRNA was much more abundant in the lamina propria than in the enterocytes, whereas brush border myosin I mRNA was restricted to the enterocytes. In the spleen, myosin IB mRNA expression was abundant in regions of white pulp, namely germinal centres, periellipsoid lymphocyte sheaths and periarteriolar lymphocyte sheaths. Lymphocytes are the major cell type in both the lamina propria and the white pulp of the spleen, which suggests that chicken myosin IB is highly expressed in lymphocytes. Lymphocyte recirculation depends on their migration through the endothelial layer and it is possible that myosin IB may have a role to play in this type of cell motility.

A 32 degree tail swing in brush border myosin I on ADP release

Brush border myosin I (BBMI) is a single-headed, unconventional myosin from intestinal microvilli, composed of a heavy chain of relative molecular mass 119,000 (M(r) 119K) and three calmodulin light chains. Although believed to have a largely structural role, it exhibits the normal actin-activated ATPase and motility properties of a member of the myosin superfamily. Here we present three-dimensional maps of BBMI-decorated actin filaments with and without bound MgADP. While the motor domain remains in a state similar to rigor, the light-chain-binding domain swings through approximately 32 degrees, resulting in a approximately 50-A movement at the end of the region visualized (the second calmodulin light chain). This could correspond to approximately 72-A movement of the entire domain. Although qualitatively similar to the movement observed in myosin II, the magnitude of the change is sufficiently different to suggest that structural changes during the actomyosin ATPase cycle differ among myosins, possibly reflecting adaptation for specialized functional demands.

myoA of Aspergillus nidulans encodes an essential myosin I required for secretion and polarized growth

We have identified and cloned a novel essential myosin I in Aspergillus nidulans called myoA. The 1,249-amino acid predicted polypeptide encoded by myoA is most similar to the amoeboid myosins I. Using affinity-purified antibodies against the unique myosin I carboxyl terminus, we have determined that MYOA is enriched at growing hyphal tips. Disruption of myoA by homologous recombination resulted in a diploid strain heterozygous for the myoA gene disruption. We can recover haploids with an intact myoA gene from these strains, but never haploids that are myoA disrupted. These data indicated that myoA encodes an essential myosin I, and this has allowed us to use a unique approach to studying myosin I function. We have developed conditionally null myoA strains in which myoA expression is regulated by the alcA alcohol dehydrogenase promoter. A conditionally lethal strain germinated on inducing medium grows as wild type, displaying polarized growth by apical extension. However, growth of the same myoA mutant strain on repressing medium results in enlarged cells incapable of hyphal extension, and these cells eventually die. Under repressing conditions, this strain also displays reduced levels of secreted acid phosphatase. The mutant phenotype indicates that myoA plays a critical role in polarized growth and secretion.

Phospholipid membrane-associated brush border myosin-I activity

Brush border myosin-I (BBMI) is associated with the membrane of intestinal epithelial cells where it probably plays a structural role. BBMI also has been identified on Golgi-derived vesicles in intestinal epithelial cells where it may translocate vesicles into the brush border. However, the mechanochemical activity of BBMI bound to a phospholipid membrane has not been described. This study reports that phospholipid membrane-associated BBMI displays ATPase activity when bound to phospholipids, but does not move actin filaments when associated with a phospholipid bilayer. BBMI does not bind significantly to brush border membrane lipids, which contain about 16% phosphatidylserine (PS), in either a pelleting or planar membrane assay. Similarly, planar membranes containing 20% PS do not bind a significant amount of BBMI. Increasing the concentration of PS to 40% does result in the binding of BBMI to both vesicles and planar membranes. This binding is enhanced with increased Ca2+ concentrations. BBMI retains its ATPase activity when bound to phospholipid vesicles containing 40% PS. However, BBMI attached to a phospholipid bilayer surface does not move actin filaments, even though the amount of BBMI bound to the lipid surface, as reflected by the number of actin filaments associated with bilayer-bound BBMI, is sufficient to observe motility in control experiments. When membrane fluidity is reduced by adding cholesterol to the membrane lipids containing 40% PS, BBMI still binds to the membrane, but again no actin filament motility is observed. The lack of binding by BBMI to brush border membrane lipids and the absence of membrane-associated BBMI mechanical activity suggest that factors in addition to membrane lipids are necessary for membrane-associated myosin-I motility.

Recombinant expression of the brush border myosin I heavy chain

Although the specific functions of myosin I motors are not known, their localization to membrane structures suggests a function in membrane motility. Different myosin I isoforms in the same cell or in different cells can possess different localizations. To determine if the localization and biochemical activity of the best-characterized mammalian myosin I, chicken intestinal epithelium brush border myosin I, was dependent on determinants of the membrane or actin cytoskeleton specific to epithelial cells, we transfected the cDNA for the heavy chain of this myosin into COS cells. Transient transfection of COS cells with the chicken brush border myosin heavy chain resulted in the production of recombinant myosin I. Recombinant brush border myosin I localized to protrusions of the plasma membrane, particularly at spreading edges, and also to unknown cytoplasmic structures. Some cells expressing particularly high levels of brush border myosin I possessed a highly irregular surface. Recombinant brush border myosin I purified from COS cells bound to actin filaments in an ATP-dependent manner and decorated actin filaments to form a characteristic appearance. The recombinant myosin also catalyzed calcium-sensitive, actin-activated MgATPase activity similar to that of the native enzyme. Thus, any cellular factor required for the general membrane localization or biochemical activity of brush border myosin I is present in COS cells as well as intestinal epithelium.

Molecular analysis of the myosin gene family in Arabidopsis thaliana

Myosin is believed to act as the molecular motor for many actin-based motility processes in eukaryotes. It is becoming apparent that a single species may possess multiple myosin isoforms, and at least seven distinct classes of myosin have been identified from studies of animals, fungi, and protozoans. The complexity of the myosin heavy-chain gene family in higher plants was investigated by isolating and characterizing myosin genomic and cDNA clones from Arabidopsis thaliana. Six myosin-like genes were identified from three polymerase chain reaction (PCR) products (PCR1, PCR11, PCR43) and three cDNA clones (ATM2, MYA2, MYA3). Sequence comparisons of the deduced head domains suggest that these myosins are members of two major classes. Analysis of the overall structure of the ATM2 and MYA2 myosins shows that they are similar to the previously-identified ATM1 and MYA1 myosins, respectively. The MYA3 appears to possess a novel tail domain, with five IQ repeats, a six-member imperfect repeat, and a segment of unique sequence. Northern blot analyses indicate that some of the Arabidopsis myosin genes are preferentially expressed in different plant organs. Combined with previous studies, these results show that the Arabidopsis genome contains at least eight myosin-like genes representing two distinct classes.

Differential calmodulin binding to three myosin-1 isoforms from liver

We have previously purified and characterized two myosin-1 isoforms from rat liver (molecular masses 130 kDa and 110 kDa; L. M. Coluccio and C. Conaty (1993) Cell Motil. Cytoskel. 24, 189–199). Here, we describe the purification and characterization from liver of a third myosin-1 (molecular mass 105 kDa) and determine the number of calmodulin molecules associated with each of these three myosin-1 isoforms. The 105 kDa polypeptide, solubilized from liver homogenates with the addition of ATP, co-sediments with F-actin, co-purifies with calmodulin, and binds calmodulin in the presence of EGTA. Antibodies directed against chicken intestinal brush border myosin-1 cross-react with the 105 kDa polypeptide on immunoblots. Partial peptide sequence analysis indicates that the polypeptide corresponds with an MM1 gamma gene product that represents a myosin-1 isoform cloned from mouse brain (Sherr et al. (1993) J. Cell Biol. 120, 1405–1416). A comparison of calmodulin binding to the now three isolated forms of myosin-1 in liver shows that in solution the 105 kDa and 110 kDa polypeptides bind two molecules of calmodulin each whereas the 130 kDa binds six molecules of calmodulin.

Rat myr 4 defines a novel subclass of myosin I: identification, distribution, localization, and mapping of calmodulin-binding sites with differential calcium sensitivity

We report the identification and characterization of myr 4 (myosin from rat), the first mammalian myosin I that is not closely related to brush border myosin I. Myr 4 contains a myosin head (motor) domain, a regulatory domain with light chain binding sites and a tail domain. Sequence analysis of myosin I head (motor) domains suggested that myr 4 defines a novel subclass of myosin I's. This subclass is clearly different from the vertebrate brush border myosin I subclass (which includes myr 1) and the myosin I subclass(es) identified from Acanthamoeba castellanii and Dictyostelium discoideum. In accordance with this notion, a detailed sequence analysis of all myosin I tail domains revealed that the myr 4 tail is unique, except for a newly identified myosin I tail homology motif detected in all myosin I tail sequences. The Ca(2+)-binding protein calmodulin was demonstrated to be associated with myr 4. Calmodulin binding activity of myr 4 was mapped by gel overlay assays to the two consecutive light chain binding motifs (IQ motifs) present in the regulatory domain. These two binding sites differed in their Ca2+ requirements for optimal calmodulin binding. The NH2-terminal IQ motif bound calmodulin in the absence of free Ca2+, whereas the COOH-terminal IQ motif bound calmodulin in the presence of free Ca2+. A further Ca(2+)-dependent calmodulin binding site was mapped to amino acids 776-874 in the myr 4 tail domain. These results demonstrate a differential Ca2+ sensitivity for calmodulin binding by IQ motifs, and they suggest that myr 4 activity might be regulated by Ca2+/calmodulin. Myr 4 was demonstrated to be expressed in many cell lines and rat tissues with the highest level of expression in adult brain tissue. Its expression was developmentally regulated during rat brain ontogeny, rising 2-3 wk postnatally, and being maximal in adult brain. Immunofluorescence localization demonstrated that myr 4 is expressed in subpopulations of neurons. In these neurons, prominent punctate staining was detected in cell bodies and apical dendrites. A punctate staining that did not obviously colocalize with the bulk of F-actin was also observed in C6 rat glioma cells. The observed punctate staining for myr 4 is reminiscent of a membranous localization.

Regulation of calmodulin binding to the ATP extractable 110 kDa protein (myosin I) from chicken duodenal brush border by 1,25-(OH)2D3

In earlier studies we observed that the active vitamin D metabolite 1,25-(OH)2D3 increased the calmodulin content of purified duodenal brush-border membrane vesicles where it bound principally to the 110 kDa protein myosin I. In this study we further evaluated the regulation of calmodulin binding to ATP releasable myosin I. Whole brush borders (BB) or purified brush-border membrane vesicles (BBMV) were prepared from duodena of vitamin D-deficient rachitic chicks treated 12-18 h before killing with either 625 pmol 1,25-(OH)2D3 or vehicle. The ATP extractable myosin I from BB resulted in an 1.6-fold increase of calmodulin binding to the 110 kDa band after treatment with 1,25-(OH)2D3. In contrast to BB, ATP extraction of myosin I from purified BBMV required alamethicin for ATP entry. As for BB extracts, calmodulin binding to the 110 kDa band in BBMV extracts was also increased about 2.4-fold by 1,25-(OH)2D3. It was concluded that both intact BB and purified BBMV showed the same type of increase in calmodulin binding to ATP releasable myosin I by 1,25-(OH)2D3. To see whether 1,25-(OH)2D3 increased the intrinsic affinity of calmodulin binding to myosin I, the ATP extractable myosin I from BB was purified from rachitic chicks treated with 1,25-(OH)2D3 or vehicle. In contrast to ATP extracts of BB or BBMV, calmodulin binding to the purified myosin I was not different between preparations from 1,25-(OH)2D3- or vehicle-treated chicks. We conclude that 1,25-(OH)2D3 does not change the affinity of calmodulin binding to myosin I but increases the amount of myosin I in the membrane or alters its ATP releasability. It was further investigated whether phosphorylation is involved in these 1,25-(OH)2D3 dependent posttranslational changes of myosin I. Phosphorylation of brush-border membrane proteins in vivo was performed by incubation of [32P]P(i) in the lumen of a ligated duodenal loop in situ for 15 min. Brush-border membrane proteins were phosphorylated in vitro by incubating BB or BBMV with [gamma-32P]ATP for 1 min. Incubation experiments in vivo and in vitro in fact resulted in phosphorylation of several proteins including 110 kDa proteins. However, there was no specific effect of 1,25-(OH)2D3 on phosphorylation of 110 kDa proteins. We conclude that the effects of 1,25-(OH)2D3 on protein phosphorylation are minimal and not likely to explain 1,25-(OH)2D3 stimulated calmodulin binding to ATP extractable brush-border membrane myosin I and 1,25-(OH)2D3 stimulated changes of calcium uptake across the brush-border membrane.

Exposure of the rat small intestine to raw kidney beans results in reorganization of absorptive cell microvilli

BACKGROUND/AIMS

A single exposure to raw kidney beans (RKB) results in vesiculation, shortening, and then regrowth of microvilli in the rat small intestine. This study investigated changes that occur in the structure of microvilli 2-10 hours after RKB exposure.

METHODS

Circumferences of microvilli from absorptive cells obtained sequentially after challenge with RKB or chow were assigned to one of three groups: small, intermediate, or large. The distribution and concentration of actin in intact mucosae or isolated epithelial sheets were determined by confocal laser scanning microscopy, immunocytochemistry, and immunoblot analysis with specific probes.

RESULTS

Six hours after exposure to RKB, most microvilli were large, abnormal in shape, and contained significantly more actin filaments than large microvilli from control rats. In addition, the fluorescence intensity of F-actin increased within injured microvilli without changes in the total intracellular actin concentration. By 8-10 hours after challenge with RKB, some microvilli remained larger than those of control rats but had resumed their normal shape and contained fewer actin filaments than at 6 hours.

CONCLUSIONS

Exposure of the rat small intestine to RKB results in enlargement of absorptive cell microvilli and reorganization of membrane and core actin filaments without changes in intracellular actin concentration. Enlarged microvilli are rapidly repaired.

Calcium-calmodulin and regulation of brush border myosin-I MgATPase and mechanochemistry

We examined the Ca(2+)-dependent regulation of brush border (BB) myosin-I by probing the possible roles of the calmodulin (CM) light chains. BB myosin-I MgATPase activity, sensitivity to chymotryptic digestion, and mechanochemical properties were assessed using 1-10 microM Ca2+ and in the presence of exogenously added CM since it has been proposed that this myosin is regulated by calcium-induced CM dissociation from the 119-kD heavy chain. Each of these BB myosin-I properties were dramatically altered by the same threshold of 2-3 microM Ca2+. Enzymatically active NH2-terminal proteolytic fragments of BB myosin-I which lack the CM binding domains (the 78-kD peptide) differ from CM-containing peptides in that the former is completely insensitive to Ca2+. Furthermore, the 78-kD peptide exhibits high levels of MgATPase activity which are comparable to that observed for BB myosin-I in the presence of Ca2+. This suggests that Ca2+ regulates BB myosin-I MgATPase by binding directly to the CM light chains, and that CM acts to repress endogenous MgATPase activity. Ca(2+)-induced CM dissociation from BB myosin-I can be prevented by the addition of exogenous CM. Under these conditions Ca2+ causes a reversible slowing of motility. In contrast, in the absence of exogenous CM, motility is stopped by Ca2+. We demonstrate this reversible slowing is not due to the presence of inactive BB myosin-I molecules exerting a "braking" effect on motile filaments. However, we did observe Ca(2+)-independent slowing of motility by acidic phospholipids, suggesting that factors other than Ca2+ and CM content can affect the mechanochemical properties of BB myosin-I.

An in vitro model for the analysis of intestinal brush border assembly. II. Changes in expression and localization of brush border proteins during cell contact-induced brush border assembly in Caco-2BBe cells

In the companion paper (M. D. Peterson and M. S. Mooseker (1993). J. Cell Sci. 105, 445–460) we describe a method for modeling brush border assembly in the Caco-2BBe clones. In this study we have examined the molecular changes accompanying cell contact-induced brush border assembly. A subset of brush border proteins was tracked throughout brush border assembly by immunoblotting and by immunofluorescent localization using laser scanning confocal microscopy. Actin, fodrin, villin and presumptive unconventional myosin immunogens were distributed at the periphery of depolarized cells. All proteins partitioned primarily with the membrane fraction upon differential sedimentation of depolarized cell lysates; the fractionation patterns were comparable to those of confluent cells. After a monolayer had formed, each protein showed a redistribution to the apical domain in a discrete sequence. Actin and villin began to shift apically at 2 d, while fodrin and the unconventional myosin immunogens did not redistribute until 3 d. Enterocyte-like localization was observed by 5 d for all proteins. Sucrase-isomaltase was not reliably detectable until 9 d by immunofluorescence, after brush border assembly was complete. Quantitative immunoblot analysis of total cell extracts demonstrated an average 10-fold increase in villin levels, while fodrin levels appeared to remain unchanged. Three putative unconventional myosin immunogens of 140 kDa, 130 kDa, and 110 kDa have been detected previously in the C2BBe cells with a head-specific monoclonal antibody to avian brush border myosin I (M. D. Peterson and M. S. Mooseker (1992) J. Cell Sci. 102, 581–600). Each of these immunogens displayed distinct expression patterns during brush border assembly. The 140 kDa species decreased by half, while the 130 kDa immunogen(s) did not change in any consistent fashion. The 110 kDa protein, presumed to be human brush border myosin I, rose on average 8-fold. A ribonuclease protection assay was also performed using a probe for human brush border myosin I. Equal amounts of total RNA from depolarized and confluent cells were assayed; the level of protected product was approximately 9-fold greater in the confluent cells. The expression patterns of the brush border proteins, coupled with the correlation to the ultrastructural features during brush border assembly in C2BBe cells, show that differentiation of the C2BBe cells closely resembles the changes that occur during human fetal intestinal differentiation.

Molecular cloning of a mouse myosin I expressed in brain

We have isolated two cDNAs that encode putative myosin I heavy chains by polymerase chain reaction amplification of brain cDNA with degenerate oligodeoxynucleotide primers representing myosin I-specific conserved amino acid sequences. We report the complete deduced amino acid sequence of one of these cDNAs. The sequences is most similar to those of the avian and bovine brush border myosin Is, with five putative calmodulin-binding repeats at the head-tail junction. Northern analysis demonstrates that this myosin heavy chain, unlike the brush border myosins, is expressed in many tissues.

Mammalian myosin I alpha, I beta, and I gamma: new widely expressed genes of the myosin I family

A polymerase chain reaction strategy was devised to identify new members of the mammalian myosin I family of actin-based motors. Using cellular RNA from mouse granular neurons and PC12 cells, we have cloned and sequenced three 1.2-kb polymerase chain reaction products that correspond to novel mammalian myosin I genes designated MMI alpha, MMI beta, MMI gamma. The pattern of expression for each of the myosin I's is unique: messages are detected in diverse tissues including the brain, lung, kidney, liver, intestine, and adrenal gland. Overlapping clones representing full-length cDNAs for MMI alpha were obtained from mouse brain. These encode a 1,079 amino acid protein containing a myosin head, a domain with five calmodulin binding sites, and a positively charged COOH-terminal tail. In situ hybridization reveals that MMI alpha is highly expressed in virtually all neurons (but not glia) in the postnatal and adult mouse brain and in neuroblasts of the cerebellar external granular layer. Expression varies in different brain regions and undergoes developmental regulation. Myosin I's are present in diverse organisms from protozoa to vertebrates. This and the expression of three novel members of this family in brain and other mammalian tissues suggests that they may participate in critical and fundamental cellular processes.

Identification, characterization and cloning of myr 1, a mammalian myosin-I

We have identified, characterized and cloned a novel mammalian myosin-I motor-molecule, called myr 1 (myosin-I from rat). Myr 1 exists in three alternative splice forms: myr 1a, myr 1b, and myr 1c. These splice forms differ in their numbers of putative calmodulin/light chain binding sites. Myr 1a-c were selectively released by ATP, bound in a nucleotide-dependent manner to F-actin and exhibited amino acid sequences characteristic of myosin-I motor domains. In addition to the motor domain, they contained a regulatory domain with up to six putative calmodulin/light chain binding sites and a tail domain. The tail domain exhibited 47% amino acid sequence identity to the brush border myosin-I tail domain, demonstrating that myr 1 is related to the only other mammalian myosin-I motor molecule that has been characterized so far. In contrast to brush border myosin-I which is expressed in mature enterocytes, myr 1 splice forms were differentially expressed in all tested tissues. Therefore, myr 1 is the first mammalian myosin-I motor molecule with a widespread tissue distribution in neonatal and adult tissues. The myr 1a splice form was preferentially expressed in neuronal tissues. Its expression was developmentally regulated during rat forebrain ontogeny and subcellular fractionation revealed an enrichment in purified growth cone particles, data consistent with a role for myr 1a in neuronal development.

PCR-dependent amplification and sequence characterization of partial cDNAs encoding myosin-like proteins in Anemia phyllitidis (L.) Sw. and Arabidopsis thaliana (L.) Heynh

Partial cDNAs encoding for myosin-like proteins from Anemia phyllitidis and Arabidopsis thaliana have been isolated using PCR technology. The deduced amino acid sequences show an average similarity up to 62% with known myosin heavy chain genes. From northern blot analysis we were able to estimate that transcripts of ca. 6.1 kb size are expressed in A. phyllitidis.

Myosin-I in mammalian liver

Myosin-I refers to a class of proteins with a molecular weight of approximately 110-kDa, which have characteristics of conventional myosin but are unable to form filaments. Previous studies have implicated myosin-I in motile cellular processes including cell migration and phagocytosis. Although the first example of myosin-I in higher eukaryotes was the intestinal 110K-calmodulin complex, which forms in microvilli the lateral links connecting the core bundle of actin filaments to the membrane, myosin-I has now been shown to be a component of rat kidney and to be present in bovine adrenal gland and brain. We have now purified and characterized two polypeptides from rat liver which have several characteristics of the intestinal 110K-calmodulin complex. Both liver polypeptides are solubilized with ATP and co-elute on gel filtration with calmodulin. The polypeptides, of 110-kDa and 130-kDa, bind calmodulin in 1 mM EGTA. Both polypeptides bind to F-actin in an ATP reversible fashion, and crosslink actin filaments. The purified polypeptides possess an actin-activated Mg(2+)-ATPase activity typical of brush border myosin-I. A polyclonal antiserum directed against the chicken intestinal 110-kDa polypeptide recognizes both rat liver polypeptides, whereas another serum recognizes the 130-kDa but not the 110-kDa rat liver polypeptide. Controlled proteolysis of the purified polypeptides with alpha-chymotrypsin indicates that the two polypeptides are distinct but related. Immunofluorescence microscopy on isolated hepatocytes shows distribution of myosin-I to be vesicular, distributed throughout the cytoplasm, but more concentrated near the nucleus. These data contribute new evidence by several functional criteria that multiple myosin-I molecules are present in higher organisms and may coexist in a single cell type.

Molecular heterogeneity of the actin filament cytoskeleton associated with microvilli of photoreceptors, Müller's glial cells and pigment epithelial cells of the retina

The present study addressed the question as to whether the four different actin-associated proteins that are associated with the actin core bundle in intestinal microvilli (i.e. villin, fimbrin, myosin I and ezrin) are essential components of all microvilli of the body. The retina provides an excellent example of a tissue supplied with three different sets of microvilli, namely those of Müller's glial cells (Müller baskets), photoreceptors (calycal processes), and pigment epithelial cells. The main outcome of this study is that none of these microvilli contain all four actin-associated proteins present in intestinal microvilli. Müller cell microvilli contain villin, ezrin and myosin I (95 kDa isoform) but not fimbrin. Calycal processes of photoreceptors contain fimbrin but not villin, myosin I and ezrin. Finally, microvilli of pigment epithelial cells are positive for ezrin but not for villin, fimbrin and myosin I. Because of limited cross-reactivities of the antibodies to myosin I and ezrin, the myosin I data refer to the chicken retina whereas the findings with anti-ezrin were obtained with the rat retina. A further outcome of this study is that the actin filament core bundles in microvilli of chicken pigment epithelial cells are presumed to contain a crosslinking protein, which is not immunologically related to either villin, fimbrin or myosin I of the intestinal brush border.

Primary structure and cellular localization of chicken brain myosin-V (p190), an unconventional myosin with calmodulin light chains

Recent biochemical studies of p190, a calmodulin (CM)-binding protein purified from vertebrate brain, have demonstrated that this protein, purified as a complex with bound CM, shares a number of properties with myosins (Espindola, F. S., E. M. Espreafico, M. V. Coelho, A. R. Martins, F. R. C. Costa, M. S. Mooseker, and R. E. Larson. 1992. J. Cell Biol. 118:359-368). To determine whether or not p190 was a member of the myosin family of proteins, a set of overlapping cDNAs encoding the full-length protein sequence of chicken brain p190 was isolated and sequenced. Verification that the deduced primary structure was that of p190 was demonstrated through microsequence analysis of a cyanogen bromide peptide generated from chick brain p190. The deduced primary structure of chicken brain p190 revealed that this 1,830-amino acid (aa) 212,509-D) protein is a member of a novel structural class of unconventional myosins that includes the gene products encoded by the dilute locus of mouse and the MYO2 gene of Saccharomyces cerevisiae. We have named the p190-CM complex "myosin-V" based on the results of a detailed sequence comparison of the head domains of 29 myosin heavy chains (hc), which has revealed that this myosin, based on head structure, is the fifth of six distinct structural classes of myosin to be described thus far. Like the presumed products of the mouse dilute and yeast MYO2 genes, the head domain of chicken myosin-V hc (aa 1-764) is linked to a "neck" domain (aa 765-909) consisting of six tandem repeats of an approximately 23-aa "IQ-motif." All known myosins contain at least one such motif at their head-tail junctions; these IQ-motifs may function as calmodulin or light chain binding sites. The tail domain of chicken myosin-V consists of an initial 511 aa predicted to form several segments of coiled-coil alpha helix followed by a terminal 410-aa globular domain (aa, 1,421-1,830). Interestingly, a portion of the tail domain (aa, 1,094-1,830) shares 58% amino acid sequence identity with a 723-aa protein from mouse brain reported to be a glutamic acid decarboxylase. The neck region of chicken myosin-V, which contains the IQ-motifs, was demonstrated to contain the binding sites for CM by analyzing CM binding to bacterially expressed fusion proteins containing the head, neck, and tail domains. Immunolocalization of myosin-V in brain and in cultured cells revealed an unusual distribution for this myosin in both neurons and nonneuronal cells.(ABSTRACT TRUNCATED AT 400 WORDS)

An unconventional myosin heavy chain gene from Drosophila melanogaster

As part of a study of cytoskeletal proteins involved in Drosophila embryonic development, we have undertaken the molecular analysis of a 140-kD ATP-sensitive actin-binding protein (Miller, K. G., C. M. Field, and B. M. Alberts. 1989. J. Cell Biol. 109:2963-2975). Analysis of cDNA clones encoding this protein revealed that it represents a new class of unconventional myosin heavy chains. The amino-terminal two thirds of the protein comprises a head domain that is 29-33% identical (60-65% similar) to other myosin heads, and contains ATP-binding, actin-binding and calmodulin/myosin light chain-binding motifs. The carboxy-terminal tail has no significant similarity to other known myosin tails, but does contain a approximately 100-amino acid region that is predicted to form an alpha-helical coiled-coil. Since the unique gene that encodes this protein maps to the polytene map position 95F, we have named the new gene Drosophila 95F myosin heavy chain (95F MHC). The expression profile of the 95F MHC gene is complex. Examination of multiple cDNAs reveals that transcripts are alternatively spliced and encode at least three protein isoforms; in addition, a fourth isoform is detected on Western blots. Developmental Northern and Western blots show that transcripts and protein are present throughout the life cycle, with peak expression occurring during mid-embryogenesis and adulthood. Immunolocalization in early embryos demonstrates that the protein is primarily located in a punctate pattern throughout the peripheral cytoplasm. Most cells maintain a low level of protein expression throughout embryogenesis, but specific tissues appear to contain more protein. We speculate that the 95F MHC protein isoforms are involved in multiple dynamic processes during Drosophila development.

Tissue distribution and subcellular localization of mammalian myosin I

Myosin I, a nonfilamentous single-headed actin-activated ATPase, has recently been purified from mammalian tissue (Barylko, B., M. C. Wagner, O. Reizes, and J. P. Albanesi. 1992. Proc. Natl. Acad. Sci. USA. 89:490-494). To investigate the distribution of this enzyme in cells and tissues mAbs were generated against myosin I purified from bovine adrenal gland. Eight antibodies were characterized, five of them (M4-M8) recognize epitope(s) on the catalytic "head" portion of myosin I while the other three (M1-M3) react with the "tail" domain. Immunoblot analysis using antiadrenal myosin I antibody M2 demonstrates the widespread distribution of the enzyme in mammalian tissues. Myosin I was immunolocalized in several cell types including bovine kidney (MDBK), rat kidney (NRK), rat brain, rat phaeochromocytoma (PC12), fibroblast (Swiss 3T3), and CHO cells. In all cases, myosin I was concentrated at the cell periphery. The most intense labeling was observed in regions of the cell usually associated with motile activity (i.e., filopodia, lamellipodia and growth cones). These results are consistent with earlier observations on protozoan myosin I that suggest a motile role for the enzyme at the plasma membrane.

Do unconventional myosins exert functions in dynamics of membrane compartments?

Unconventional myosins have now been identified in amoeba as well as in higher eucaryotic cells. Their cellular localization, their ability to bind membrane vesicles and their ability to produce in vitro movement suggest that they can generate forces on the plasma membrane relative to actin filaments as well as on membrane compartments relative to actin. Genetic approaches and biochemical analysis of cells over-producing nonfunctional domains of unconventional myosins have provided direct evidence for a role of unconventional myosins in movement of intracellular vesicles and have allowed us to formulate hypotheses about the possible mechanisms by which unconventional myosins could participate in the intracellular transport of membrane proteins and secretory proteins.

Localization and specificity of the phospholipid and actin binding sites on the tail of Acanthamoeba myosin IC

We used bacterially expressed beta-galactosidase fusion proteins to localize the phospholipid binding domain of Acanthamoeba myosin IC to the region between amino acids 701 and 888 in the NH2-terminal half of the tail. Using a novel immobilized ligand lipid binding assay, we determined that myosin I can bind to several different acidic phospholipids, and that binding requires a minimum of 5 mol% acidic phospholipid in a neutral lipid background. The presence of di- and triglycerides and sterols in the lipid bilayer do not contribute to the affinity of myosin I for membranes. We confirm that the ATP-insensitive actin binding site is contained in the COOH-terminal 30 kD of the tail as previously shown for Acanthamoeba myosin IA. We conclude that the association of the myosin IC tail with acidic phospholipid head groups supplies much of the energy for binding myosin I to biological membranes, but probably not specificity for targeting myosin I isoforms to different cellular locations.