Guide Myosins: A Superfamily of Molecular Motors

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Myosins as fundamental components during tumorigenesis: diverse and indispensable

Seller Inventory AAV Book Description Springer, New Book. Except for the IQ and coiled-coil domains, the SMART program used to identify the motor domain of the myosin sequences did not identify any domains other than a few with scores less significant than the required threshold. Myosins have highly conserved residues spread throughout the motor domain that define a core consensus sequence [ 26 ]. Comparison of an alignment of Arabidopsis myosin motor domains to these conserved sequences shows a great deal of conservation among them data not shown.

With the exception of only one residue these are conserved in all 17 Arabidopsis myosins. The plant sequences are very conserved through this region. Cope et al. The presence of these highly conserved residues in plant myosins suggests that they are capable of motor function. In fact, in vitro motility studies with a purified myosin from Chara myosin XI, Cc ccm in Figure 3 have confirmed that it is indeed an actin-based motor [ 54 ]. A loop present in the motor domain called the HCM mutations in this loop cause hypertrophic cardiomyopathy is the location of a phosphorylatable serine S or threonine T in certain amoeboid myosin I molecules and myosin VI molecules.

The enzyme activity of the amoeboid myosins depends on phosphorylation of this site, but although phosphorylation of the myosin VI T residue has been demonstrated, the regulation of enzyme activity has not [ 8 , 63 ]. Most other myosins have a constitutively negatively charged amino acid, either aspartic acid D or glutamic acid E at this site. This site has been named the TEDS rule site on the basis of these amino acids [ 8 ]. The Arabidopsis and other plant myosins all have aspartic acid, glutamic acid or glycine residue at this site, suggesting that they are not regulated by phosphorylation at this site.

The site for each predicted or actual intron was located and is shown schematically in Figure 6. The intron locations were determined from the information at MIPS [ 57 ]. The exons vary in length from 12 to greater than nucleotides the length of the beginning and last exons for each gene are not known as the predicted sizes include only the protein-coding nucleotides with an average of nucleotides.

The four class VIII myosins have seven exons of the same length in the same order within the myosin motor domain Table 2. The motor domain starts in the third exon of each class VIII myosin. The class XI myosins also have many exons that are of the same length and in the same order but that differ from the class VIII pattern Table 3. The exons coding for the motor domain sequence are most conserved in length. Most class XI myosins motor domains start in the third exon and end in the twentieth. Six of the class XI myosins have an intron after the start codon.

Most differences in exon length are in the carboxy-terminal regions Figure 6 and Table 3. However, even in the carboxy-terminal region there are some exon lengths conserved between some or all of the myosins. Their motor domains are Twenty-three of their introns are at the same location in the motor domain area and then following a few different size exons, there are similar sized exons again.

They are located on chromosomes I and V, respectively. It is possible that this pair is a result of gene duplication. This again may have resulted from a gene duplication. Analysis of the total Arabidopsis genome revealed that a whole genome duplication occurred, followed by subsequent gene loss and extensive local gene duplications [ 55 ].

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The S. Location of the introns. Arrowheads indicate the location of each intron along the length of the myosin. If the gene pairs are the result of duplication, it is interesting to note that while exon lengths have been conserved, intron lengths have not. The intron lengths are shown in Table 4. No pattern can be seen in intron lengths between any of the myosins. The average intron length is nucleotides with the shortest intron at 47 nucleotides and the longest at At XI-I has the highest average, nucleotides.

It contains the nucleotide intron and three others that are over nucleotides. In a study of introns only 3. Only two other myosins had an intron over nucleotides. Hunt et al. Several of the introns in the myosins are between 66 and 70 nucleotides and so may be long enough to be spliced. Only one is in a cloned myosin known to be spliced at that site At XIJ. There is also a predicted intron of only 47 nucleotides in length At XID which is thought to be too short for efficient splicing.

Brown et al. Until the expression of At XID is studied, no conclusion can be made as to the validity of this intron prediction. The significance of the range and variability of intron length is not known. In Arabidopsis, in general, the range is even greater , [ 11 ]. The splice sites in the reported myosins and the predicted myosins Table 4 all contain the 5' GT and 3' AG sequences.

The sequences in the Arabidopsis myosins upstream and downstream of these two very conserved sites varied as a reflection of the less conserved nature of these nucleotides Table 4. However, these predicted sites at the 5' and 3' splice sites need to be confirmed experimentally.

Only two classes of myosins are present in Arabidopsis. A study of myosins in lily and tobacco pollen tubes using antibodies to three animal-type myosins IA and IB, II and V suggested the presence of three types of myosins in these plants [ 40 ]. Class XI are somewhat similar to class V myosins [ 42 ] and this may explain the reaction with the type V antibody. Possibly the other reactions were due to similarities in the myosin motor domain.

Phylogenetic analysis of Arabidopsis myosins along with other plant myosins suggests that most class XI myosins except three fall into two subgroups Figure 4. The Arabidopsis myosins have anywhere from three to six IQ domains. The IQ domain in non-plant myosins has been shown to bind to calmodulin in a calcium-independent manner.

The regulation of myosin action is thought to be due to calmodulin interaction. In plants, two myosin heavy chains have been shown to associate with calmodulin [ 37 , 67 ]. A myosin-containing protein fraction from tobacco BY2 cells was used in motility assays with F-actin. The myosins in the above studies have not been cloned, and binding to specific IQ domains has not been established. It would be interesting to investigate the possible phosphorylation of the threonine residue which is three residues upstream from the TEDS rule site in class XI myosins and to see if enzyme activity is regulated by phosphorylation of this residue.

Myosins are involved in a wide range of cellular functions. They have been shown to be involved in movement, translocation, cell division, organelle transport, G-protein-linked signal cascade and maintenance of structure within cells [ 26 ]. Insight into the function of plant myosins has been gained by studies in algae.

Cytoplasmic streaming is responsible for movement of organelles and vesicles and of generative cells and vegetative nuclei in pollen tubes. A myosin isolated from the alga Chara corallina was shown to be responsible for cytoplasmic streaming [ 30 , 69 , 70 ]. The myosin was cloned and characterized and found to be a class XI myosin related to the Arabidopsis MYA myosins [ 54 ].

Myosins in plants have also been shown to be involved in cytoplasmic streaming. Using immunofluorescence, myosin was localized to vesicles, organelles and generative cells and vegetative nuclei in grass pollen tubes [ 39 ].

Myosin and actin - Circulatory system physiology - NCLEX-RN - Khan Academy

A myosin isolated from lily pollen has been shown to be responsible for cytoplasmic streaming in pollen tubes and two myosins were identified in tobacco cell cultures that are also thought to participate in cytoplasmic streaming [ 37 , 71 ]. Antibodies to the myosins recognized a protein in vegetative cells as well as pollen tubes.

Liu et al. Previous studies showed that translocational step size produced by a myosin motor is proportional to the number of IQ domains and the larger the step the faster or more efficiently they are able to transport vesicles [ 9 ]. However, the kinetic properties of the motor domain are also involved in speed and there is a wide range of movement speeds for myosin II molecules [ 2 , 72 , 73 ].

An antibody specific to a Z. The root tip cells showed particulate staining in the cytoplasm, but neither the vacuole membrane nor plasma membrane were stained, although in some cells the staining was too bright to distinguish if the plasma membrane was stained or not.


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There are 13 class XI myosins in Arabidopsis that could be involved in vesicle and organelle transport. The large number could reflect redundancy of function or differential expression. Patterns of expression were different for the cloned Z. Immunolocalization studies have also detected myosin associated with plasmodesmata.

Plasmodesmata are interconnections between contiguous plant cells that allow direct cell-to-cell transport of ions and proteins. Earlier work suggested that actin was involved in regulation of plasmodesmal transport [ 74 ]. Other studies using antibodies to animal myosins in root tissues of Allium cepa, Z. However, immunolocalization studies with antibodies to animal myosins need to be interpreted with caution as there are no plant myosins that group with animal myosins. The recent work by Reichelt et al.

The myosin was localized mainly to the transverse walls with some punctate labeling of the longitudinal walls. During cell division the anti-class-VIII myosin staining remains confined to the transverse cell walls and is strongest in the newly formed cell wall. Immunogold electron microscopy showed labeling of class VIII myosin associated with the plasma membrane and plasmodesmata. These studies suggest that class VIII myosins may be involved in new cell wall formation and transport in the plasmodesmata.

Reichelt et al. The role of myosin in the plasmodesmata was studied further by pretreating tissue with 2,3-butanedione 2-moxoxime BDM , an inhibitor of actin-myosin motility. The pretreatment resulted in a strong constriction of the neck region of plasmodesmata [ 38 ]. Myosin VIII in the plasmodesmata could be a part of a gating complex that is thought to control the opening of the plasmodesma neck [ 74 ].

A recent study of the effect of BDM on the distribution of myosins, F-actin, microtubules and cortical endoplasmic reticulum ER suggests that myosins may link together microtubules and actin filaments involved in structural interactions [ 75 ]. BDM treatment disrupted normal cellular distributions of maize myosins and the characteristic distribution of F-actin was also affected. Myosin may participate in the intracellular distribution of actin filaments as was proposed for myosin XV [ 76 ]. Microtubule arrangements in cortical root cells were altered, as was the normal ER network.

Post-mitotic cell growth was inhibited by BDM, specifically in the transition zone and the apical parts of the elongation region. The study suggested that actin fibers and microtubules interact together via myosins and that myosin-based contractility of the actin cytoskeleton is essential for the developmental progression of root cells [ 75 ].

However, BDM has only been shown to inhibit a few myosins in vitro [ 77 ] and is known to be a nonspecific inhibitor; so these results must be viewed with caution. As the classification system of myosins now stands, plant myosins fall only into two classes - class VIII and class XI. All animal cells examined contain at least one myosin II gene and usually multiple myosin I genes [ 8 ], but this is not true for Arabidopsis specifically and possibly for all plants. Krizek, J. FEBS Mapping the microvillar K-calmodulin complex: Calmodulin-associated or -free fragments of the kd polypeptide bind F-actin and retain ATPase activity.

Bullitt, E. DeRosier, L. Coluccio, and L. Three-dimensional reconstruction of an actin bundle. Reassociation of microvillar core proteins: Making a microvillar core in vitro.

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Mapping of the microvillar K-calmodulin complex brush border myosin 1. Identification of fragments containing the catalytic and F-actin-binding sites and demonstration of a calcium ion dependent conformational change.

Oncotarget | Myosins as fundamental components during tumorigenesis: diverse and indispensable

Biochemistry Identification of the microvillar kDa calmodulin complex myosin-1 in kidney. Myosin-I in mammalian liver. Cell Motil. Williams, R. Novel kDa rat liver myosin-I will translocate actin filaments. Differential calmodulin binding to three myosin-I isoforms from liver. Cell Sci. An end in sight: Tropomodulin. Invited mini-review J. Balish, M. Identification of brush border myosin-I in liver and testis. Phosphorylation of myosin-I from rat liver by protein kinase C reduces calmodulin binding. Veigel, C. Coluccio, J. Jontes, J. Sparrow, R. Milligan, and J.

Myosin-I produces its working stroke in two steps. Nature Transient kinetic analysis of the kDa myosin I myr 1 gene product from rat liver: A myosin I designed for maintenance of tension? Moeller, III, and L. Overlapping distribution of the and kDa myosin I isoforms on rat liver membranes. Geeves, M. Invited conference report. Muscle Research and Cell Motility. Li, W. Wang, L. Coluccio, P. Matsudaira and R. Brush border myosin I: A basally localized transcript in human jejunal enterocytes. Perreault-Micale, C. Shushan and L. Perreault-Micale, and L. Kinetic analyses of a truncated mammalian myosin I suggest a novel isomerization event preceding nucleotide binding.

Wallace, M. Batters, L. Coluccio and J. Nanometre resolution tracking of myosin-1b motility. IEE Proc. Batters, C. Arthur, A. Lin, J. Porter, M. Geeves, R. Milligan, J.