Nonmuscle Myosin II

Nonmuscle Myosin II

Myosin II plays an important role in a wide variety of contractile events in eukaryotic cells by supporting ATP-dependent movement of actin filaments. The functional myosin II protein i= s a hetero-hexamer comprised of two ‘heavy’ chains, two regulatory light chains, and two essential light chains. The globular “head” do= mains of the myosin II heavy chains (MHC) drive the movement of actin filaments v= ia conformational changes induced by actin binding, ATP hydrolysis, and the subsequent release of ADP and inorganic phosphate. In the intact molecule, two MHC he= ads are positioned next to each other via the formation of coiled-coils between= the extended “tail”’ regions of each heavy chain (Figure 1).<= span style=3D'mso-spacerun:yes'> Interactions between the tail doma= ins of individual myosin II molecules lead to the formation of bipolar “thick” filaments and enables myosin II to contract opposing ac= tin filaments (Figure 2).

There are two major proc= esses controlling myosin II-mediated contraction of actin filaments: 1) regulatory light chain (RLC) phosphorylation and 2) assembly of myosin II molecules into anti-para= llel bipolar filaments. Phosphoryl= ation of the RLC stimulates myosin II-mediated movement of actin filaments by increasing the actin-activated ATPase activity of the myosin II head. By contrast, bipolar filament asse= mbly is a prerequisite for myosin II-driven contraction of actin filaments; thus= the regulation of filament assembly has a profound impact on cellular contraction. In the context o= f the skeletal muscle cell, myosin II bipolar filaments are arranged into stable, ordered arrays between actin filaments and the mechanisms by which myosin m= ediates myofibril contraction have been elucidated (1). By comparison, myosin II in non= muscle cells undergoes a dynamic assembly/disassembly process that permits its specific spatial and temporal localization within the cell (2). However, the mechanisms regulating= the assembly and localization of myosin II filaments in nonmuscle cells are not well understood (3).

Studies in the social amoeba Dictyostelium discoideum have revealed that complex cellular processes such as cytokinesis, cellular migration, maintenance of cell cortical tension, cell surface receptor capping, and multicellular development depend on the proper function and regulation of myosin II (4). The diversity of these myosin-dependent processes indicates that cells must be able to undergo hig= hly localized reorganization of their myosin II assemblies in response to signa= ls initiating specific contractile processes (3,4). For example, myosin II bipolar fil= aments accumulate transiently at the cleavage furrow of dividing cells where they contribute to the separation of the two daughter cells via localized contraction of actin filaments (5-7). In another context, myosin II fila= ments localize to the posterior of migrating cells where they contribute to direc= ted cell movement by facilitating retraction of the cell rear and suppressing lateral pseudopod formation (5,8-10).

Regardless of the cellul= ar process or its localization in the cell, myosin II must first assemble into bipolar filaments before it can mediate the contraction of actin filaments = (11). This requirement highlights the importance of understanding not only the basic processes controlling myosin= II filament assembly, but also how variations in such regulatory mechanisms can ultimately specify which of the diverse roles myosin II plays in the ever-changing environment of a nonmuscle cell. Extensive biochemical and in vivo studies have shown that Dictyostelium myosin II exists in a dynamic equilibrium between a cytoplasmic pool of monomers and a cytoskeleton-associated assembly of bipolar filaments (Figure 2) ADDIN EN.CITE <EndNote><Cite><Author>de la Roche</Author><Year>2001</Year><RecNum>31</RecNu= m><record><rec-number>31</rec-number><ref-type name=3D'Journal Article'>17</ref-type><contributors><authors><autho= r>De la Roche, M. A.</author><author>Cote, G. P.</author></= authors></contributors><auth-address>Department of Biochemistry, Queen's University, K7L 3N6, Kingston, Ont., Canada.</auth-address><titles><title>Regulation of Dictyostelium myosin I and II</title><secondary-title>Biochim Biophys Acta</secondary-title></titles><periodical><fu= ll-title>Biochim Biophys Acta</full-title></periodical><pages>245-61.</= pages><volume>1525</volume><number>3</number><= ;keywords><keyword>Actins/metabolism</keyword><keyword>= ;Animal</keyword><keyword>Ca(2+)-Calmodulin Dependent Protein Kinase/metabolism</keyword><keyword>Dictyostelium/genetics/*met= abolism</keyword><keyword>Microfilaments/metabolism</keyword= ><keyword>Models, Molecular</keyword><keyword>Myosins/genetics/*metabolism</ke= yword><keyword>Protein Isoforms/metabolism</keyword><keyword>Signal Transduction</keyword><keyword>Support, Non-U.S. Gov't= </keyword><keyword>src Homology Domains</keyword></keywords><dates><year>2001</y= ear></dates><accession-num>11257438</accession-num><= ;urls><related-urls><url>http://www.ncbi.nlm.nih.gov/htbin-p= ost/Entrez/query?db=3Dm&form=3D6&dopt=3Dr&uid=3D112= 57438</url></related-urls></urls></record></Cite= ><Cite><Author>Spudich</Author><Year>1995</Ye= ar><RecNum>43</RecNum><record><rec-number>43<= /rec-number><ref-type name=3D'Journal Article'>17</ref-type><contributors><authors><autho= r>Spudich, J. A.</author><author>Finer, J.</author><author>Simmons, B.</author><author>Rupp= el, K.</author><author>Patterson, B.</author><author>Uy= eda, T.</author></authors></contributors><auth-address>D= epartment of Biochemistry, Stanford University, California 94305, USA.</auth-address><titles><title>Myosin structure and function</title><secondary-title>Cold Spring Harb Symp Quant Biol</secondary-title></titles><periodical><full-title= >Cold Spring Harb Symp Quant Biol</full-title></periodical><pages&= gt;783-91</pages><volume>60</volume><keywords><k= eyword>Amino Acid Sequence</keyword><keyword>Animal</keyword><keyword>= ;Biomechanics</keyword><keyword>Dictyostelium/genetics/physiolo= gy</keyword><keyword>Genetics, Biochemical</keyword><keyword>Lasers</keyword><keyword= >Models, Molecular</keyword><keyword>Molecular Sequence Data</keyword><keyword>Molecular Structure</keyword><keyword>Movement/physiology</keyword>= <keyword>Muscle Contraction/physiology</keyword><keyword>Mutation</keyword&g= t;<keyword>Myosins/*chemistry/genetics/*physiology</keyword><= ;keyword>Protein Conformation</keyword><keyword>Support, Non-U.S. Gov't</keyword><keyword>Support, U.S. Gov't, P.H.S.</keyword></keywords><dates><year>1995</ye= ar></dates><accession-num>8824453</accession-num><u= rls><related-urls><url>http://www.ncbi.nlm.nih.gov/htbin-pos= t/Entrez/query?db=3Dm&form=3D6&dopt=3Dr&uid=3D88244= 53</url></related-urls></urls></record></Cite>= ;</EndNote>(4,12). The ability of Dictyostelium myosin= II to assemble into filaments is inhibited dramatically upon phosphorylation of three specific threonine residues (sites 1823, 1833, and 2029) located toward the carboxy-= terminus of the MHC tail (13-16). These MHC phosphorylation sites we= re initially recognized as target sites for the enzyme MHC kinase A (MHCK A); studies of this enzyme are a major component of this grant proposal. In later studies, the importance of these target sites in controlling cellular myosin filament assembly was demonstrated by showing that conversion of these threonines to aspartic acid residues creates a “pseudo-phosphorylated” myosin II (3x ASP-myosin) that is unable to assemble into the cytoskeleton (16). Cells expressing 3X ASP-myosin are phenotypically identical to MHC null cells with defects in both cytokinesis= and multicellular development (16). Conversely, myosin II over-assembl= es into the cytoskeleton if these regulatory sites are either removed by truncation of MHC tail (17) or converted to non-phosphorylatable alanine residues (16). Taken together, these studies, as = well as others not mentioned here, have established that MHC phosphorylation is = the main route by which myosin II filament assembly and localization is regulat= ed in Dictyostelium (4).

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Ref= erences

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