Tropomyosin: Difference between revisions

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Alexander Berchansky, Michal Harel, David Canner, Jaime Prilusky, Gregory Hoeprich, Joel L. Sussman

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-[[Image:1c1g.png|left|150px|thumb|Crystal Structure of Pig Tropomyosin, [[1c1g]]]]
+<StructureSection load='' size='350' side='right' scene='User:Gregory_Hoeprich/Sandbox_1/Tropomyosin_dimer/2' caption='Pig tropomyosin (PDB code [[1c1g]])'>
-{{STRUCTURE_1c1g|  PDB=1c1g  | SIZE=300| SCENE=Tropomyosin/Cv/1 |right|CAPTION=Tropomyosin from pig, [[1c1g]] }}
+[[Image:B Lehman1.jpg | thumb | 300 x 430px | left | alt text | '''Tropomyosin''' (seen in yellow and red) wrapped around actin filaments, which are EM reconstructions with G-actin ribbion structures filling in the EM structure. (Picture generated from William Lehman's [http://www.bumc.bu.edu/phys-biophys/research/filhel/ Website]) ]]
+__TOC__
-[[Tropomyosin]] '''(TPM)''' has a 4-helix coiled structure.  It regulates the binding of [[Myosin|myosin]] thus regulating muscle contraction <ref>PMID:9108196</ref>. In its locked conformation it binds [[Troponin|troponin]] T (TnnT) and prevents the binding of myosin to actin. When Ca++ ions bind to TnnT, the TPM assumes an open conformation and myosin can bind to actin. The images on the left and the right correspond to one representative TPM structure, ''i.e.'' tropomyosin from pig ([[1c1g]]). You can  <scene name='Tropomyosin/Cv/2'>enlarge the image</scene> at the right for clarity. The dimers of TPM in the asymmetric unit ([[1c1g]]) are <scene name='Tropomyosin/Cv/6'>antiparallel</scene>, with their C-terminal ends overlapping by about 2/3 of the molecular length. This suggests head-to-tail packing of TPM, which is very important for its interaction with actin <ref>PMID:10651038</ref>.
+==Function==
+'''[[Tropomyosin]] (TM)''' is an [[actin]] binding protein, which consists of a coiled-coil dimer (see left) and forms a polymer along the length of actin by a head-to-tail overlap along the major grove of actin (see down & left)<ref name="Gunning">Tropomyosins. I. Gunning, Peter, 1950- II. Series.[DNLM: 1. Tropomyosin. W1 AD559 v.644 2008 / WE 500 T856 2008]</ref>.  See [[Coiled coil]].  The head-to-tail overlap allows flexibility between the tropomyosin dimers so it will lay unstrained along the filament<ref name="Gunning"/>.  Each tropomyosin molecule spans seven actin monomers within a filament and lays N- to C- terminally from actin's pointed to barbed end<ref name="Frye">PMID:20465283</ref>.  The 284 amino acid helix has a length of 420 Angstroms and has a molecular weight around 65-70 kilodaltons (vertebrate tropomyosin)<ref name="Gunning"/><ref name="Whitby">PMID:10651038</ref>.  A few of tropomyosin's characteristics as an actin binding protein includes regulation, stabilization and recruitment.
+<br />
+Its role in muscle mechanics has been well established as it is a major regulatory component of the contractile apparatus, but its role in non-muscle systems is becoming evermore clear.  In mammals, there are at least 40 known isoforms, which are generated by alternative splicing of multiple genes<ref name="Gunning"/><ref name="Frye"/>.  These isoforms in non-muscle systems contribute to actin's stability by increasing filament rigidity and protecting the filament from actin severing proteins, like [[gelsolin]] or [http://en.wikipedia.org/wiki/Cofilin cofilin]. The different isoforms also aid in recruitment of various proteins, including myosin (a family of molecular motors)<ref name="Clayton"/><ref name="Stark"/>.  The reason for tropomyosin's diversity is not well known, but it is thought to exist to function at different developmental stages in some species as well as function in specific cells of higher multicellular organisms<ref name="Gunning"/>.
+<br />
 <br />
+The details of its structure/function relationship defines its role in different cell systems and disease states, this page further discuss tropomyosin's structure and function.  See also [[Fibrous Proteins]].
-{{TOC limit|limit=2}}
+== Tropomyosin's Structure ==
+[[Image:Tropomyosin Alpha helix.png | thumb | left | 250x380px | alt text | '''Tropomyosin's''' Alpha Helices are represented in ribbon (Chain A) and ball and stick form (Chain B, without side chains).  The yellow lines represent the polar contacts within the protein backbone, which stabilize the helical structure.]]
-== 3D Structures of Tropomyosin ==
+{{clear}}
+As determined by the '''S'''tructural '''C'''lassification '''o'''f '''P'''rotein's ([http://scop.mrc-lmb.cam.ac.uk/scop/ '''SCOP''']) Database, Tropomyosin is categorized as follows (general to specific):
+#'''Class:''' coiled-coil
+#'''Fold:''' parallel coiled-coil
+#'''Superfamily:''' tropomyosin
+#'''Family:''' pig [[http://www.pdb.org/pdb/explore/explore.do?structureId=1C1G 1c1g]]
+[[Image:Helical Wheel Respresentation of Tropomyosin.jpg | thumb | right | 333x200px | alt text | '''Helical Wheel Diagram Representation of Tropopomyosin:'''  The coiled-coil dimer is stabilized by hydrophobic and ionic interactions (red=hydrophobic, blue=polar & green=charged)Image Reconstructed from <ref name="Gunning"/>.]]
+{{clear}}
+Categorization of tropomyosin's <scene name='User:Gregory_Hoeprich/Sandbox_1/Tropomyosin_dimer/2'>coiled-coil</scene> describes a unique pattern of amino acids within the primary structure of the alpha helices that comprise the dimer interface<ref name="Gunning"/>.  The unique amino acid pattern, found within all coiled-coil proteins, is a heptad repeat, which follows a similar pattern to: '''H-P-P-H-C-P-C''', where H is hydrophobic, P is polar and C is charged<ref name="Gunning"/><ref name="Whitby"/>.  This heptad repeat forms a right-handed alpha helical secondary structure (see right for alpha helix secondary structure)<ref name="Gunning"/>.  This alpha helix is special in that it forms a hydrophobic strip along one side, which will interface with an adjacent alpha helix that also contains the heptad repeat and hydrophobic strip.  These strips aid in the dimerization of tropomyosin and is important in the characteristic coiled-coil domain.
 <br />
+This coiled-coil domain is represented as a helical wheel diagram (see right), whereby the four amino acids (two from each alpha chain) that are adjacent to each other contribute to a <scene name='User:Gregory_Hoeprich/Sandbox_1/Tropomyosin_hydrophobic_aa/4'>hydrophobic interaction</scene>, represented in red, while the four amino acids (two from each alpha chain) flanking the hydrophobic core provide an ionic interaction, or salt-bridge represented as green.  The <scene name='User:Gregory_Hoeprich/Sandbox_1/Tropomyosin_ionic_interaction/2'>salt-bridge</scene> can be observed by observing chain A, residue 26, which is a glutamic acid and forms a salt bridge with an arginine, 305 on Chain B.   This weak stabilizing interaction has both and electrostatic interaction, via the ions, and two hydrogen bonds between the residues.  This extra stability aids in its left-handed super-coil conformation.  The two hydrogen bonds between the residues have an average distance of 2.85Å.  Combined the hydrophobic and ionic interactions contribute significantly to the stability of the dimer<ref name="Gunning"/><ref name="Whitby"/>.
 <br />
 <br />
+The amino acids from each alpha chain interact with each other uniquely in that side chains of the helices interfacing will pack together in a knobs in holes arrangement (''i + i+4'' ridges fit into ''i + i+4'' grooves)<ref name="Gunning"/>.  This is accomplished by the ridges of one helix fitting between the grooves (between the hydrophobic amino acids in position a & d, see helical wheel diagram) of the adjacent helix.  This packing further allows for the two alpha helices to wrap tightly around each other in a left handed supercoil conformation further stabilizing the tropomyosin dimer<ref name="Gunning"/>.
 <br />
 <br />
+Tropomyosin, as mentioned above, will form a long polymer along the length of actin in a head-to-tail overlap<ref name="Frye"/>. This <scene name='User:Gregory_Hoeprich/Sandbox_1/Tropomyosin_overlap/2'>overlap region</scene> occurs as the amino acids from the <scene name='41/410306/Tropomyosin_overlap_c_d/1'>N-terminus</scene> of one dimer overlaps with the amino acids of the <scene name='41/410306/Tropomyosin_overlap_a_b/1'>C-terminus</scene> of another dimer.  There are several <scene name='41/410306/Tropomyosin_overlap_a_b_e_f/1'>intermolecular contacts</scene> in the overlap region, which consist of ionic, hydrophobic and non-polar interactions<ref name="Frye"/>.  (Note: many of the methionine residues interacting between the dimers are selenomethionine, which are used here to help solve the x-ray crystal structure.)  Interestingly, most of the variation seen among tropomyosin isoforms is in the overlap region, which will affect polymer formation along the actin filament<ref name="Frye"/>.  The actin binding sites on tropomyosin (protein-protein interactions) are currently not easily recognizable, but it is thought that a periodic repeat of seven consensus residues contribute to tropomyosin binding to actin<ref name="Gunning"/>.  The seven fold repeat is  a mix of charged and non-polar residues in the 2,3 and 6 positions (see helical wheel diagram above)<ref name="Gunning"/>.
 <br />
+== Post-Translational Modifications ==
+There are two types of post-translational modifications to tropomyosin: phosphorylation acetylation<ref name="Gunning"/>.  Phosphorylation occurs on amino acid <scene name='41/410306/Tropomyosin_dimer_ser229/1'>Ser-229</scene><ref name="Gunning"/>.  This phosphorylation is occurs as a result of oxidative stress, which is associated with actin remodeling and recruitment of additional tropomyosin into stress fibers<ref name="Gunning"/>.  The acetylation occurs on the N-terminus  of the N-terminal methionine, which is essential for: coiled-coil stability, overlap formation and actin binding<ref name="Frye"/><ref name="Gunning"/>.
+=== Evolutionary Conservation ===
+Tropomyosin is highly conserved actin binding protein, which is found in Eukarya from the animal kingdom to yeast, with the exception of plants<ref name="Gunning"/>.  The earliest characterization of tropomyosin gene lineage was in yeast (budding and fission yeast)<ref name="Gunning"/><ref name="Drees">PMID:7844152</ref>.  These genes are known as [http://en.wikipedia.org/wiki/TPM1 TPM1] and [http://en.wikipedia.org/wiki/TPM2 TPM2], respectively, and share 64.5% sequence identity<ref name="Gunning"/><ref name="Drees"/>.  As we move away from unicellular organisms and into multicellular invertebrates, tropomyosin genes in nematodes slightly diverge from yeast, but have 85-90% sequence identity between their genes<ref name="Gunning"/>.  Further analysis for vertebrates show there are four genes that generate over 40 known mammalian isoforms of tropomyosin, which are synthesized by exon splicing<ref name="Gunning"/>.  The slight evolution change of tropomyosin has occurred as a result of the increasing need of tropomyosin to function in different systems, but tropomyosin has evolutionarily stayed well conserved because of the basic structural pressures imposed on the protein<ref name="Gunning"/>.  It is interesting to note, the region with the least conservation has been in the N and C terminus.  This is the result of head-tail interactions changing to accommodate different polymer confirmations along actin for different functions<ref name="Gunning"/>.  (To see this evolutionary conservation, go to the top right image of the web page and click on the "show" link, which is to the right of "Evolutionary Conservation".)
+== Tropomyosin's Function: Muscle and Non-Muscle Systems ==
+Muscle tissue is comprised of many muscle fibers or cells.  Those muscle fibers consist of myofibrils that contain a series of contractile units called sarcomeres.  Within this unit contains thick filaments, comprised mainly of [[myosin]], and thin filaments, which contain [[actin]], tropomyosin and [[troponin]].  Tropomyosin in striated muscle systems (skeletal and cardiac) acts to inhibit the myosin cross-bridges from binding to the myosin binding site on thin filaments, this tropomyosin state is in the "Blocked" position<ref name="Lehman">PMID:19341744</ref>.  When the muscle is stimulated, there is a rise in intracellular calcium stemming from a cascade of cellular processes.  As the calcium is bathing the sarcomere, it will bind to the troponin complex, which is bound to both actin and tropomyosin<ref name="Tyska">PMID:11810692</ref>.  The troponin will displace the tropomyosin from a "Blocked" to a "Closed" position<ref name="Lehman"/>.  This transition allows the myosin head to interact weakly with the myosin binding site<ref name="Tyska"/>.  The tropomyosin is displaced to its final position, "Open" state, along actin filament as myosin binds to its site<ref name="Lehman"/>.  These three tropomyosin states along the filament is referred to as the three state model<ref name="Lehman"/>. As the intracellular calcium concentration falls, the troponin no longer is able to displace tropomyosin and it will transition back to the "Blocked" state<ref name="Lehman"/>.
 <br />
-[[3mtu]], [[3mud]] – cTPM alpha-1 – chicken<br />
+<br />
-[[1ic2]] - cTPM alpha-1 (mutant)<br />
+In non-muscle systems, as mentioned above, tropomyosin performs a wide variety of functions inside of cells.  One of the specific functions tropomyosin performs within cells is recruitment of different myosin motors.  Clayton ''et al'', 2010 showed actin decorated with tropomyosin increased myosin-V (cargo transporter) motility as opposed to bare actin<ref name="Clayton">PMID:20705471</ref>.  It was also shown that myosin-I (cargo transporter and tension senor) lost its motility function on actin decorated with tropomyosin<ref name="Clayton"/>.  Further evidence from a related paper, Stark ''et al'' 2010, shows tropomyosin increases the affinity of myosin-II to actin filaments<ref name="Stark">PMID:20110347</ref>.  So actin decorated with tropomyosin appears to spatially regulate motor activity and thus cellular function.
-[[2w49]], [[2w4u]] – cTnnC+cTnnT+cTnnI+cTPM alpha-1+cActin<br />
-[[2z5h]] – yTPM alpha-1 N-terminal+C-terminal+GNC4 leucine zipper+TnnT – yeast<br />
+== 3D structures of Tropomyosin  ==
-[[2z5i]] - yTPM alpha-1 N-terminal+C-terminal+GNC4 leucine zipper<br />
-[[2efr]], [[2efs]], [[2d3e]] - rTPM alpha-1 C-terminal+GNC4 leucine zipper – rabbit<br />
+[[Tropomyosin 3D structures]]
-[[1kql]] - TPM alpha-1 C-terminal+GNC4 leucine zipper - rat<br />
-[[1mv4]] - TPM alpha-1 C-terminal – rat<br />
-[[2g9j]] - TPM alpha-1 TM9A+GNC4 – rat<br />
-[[2b9c]] – TPM mid region – rat<br />
-[[1c1g]] – TPM – pig<br />
-[[2tma]] – TPM - model<br />
 ==References==
 <references />
+</StructureSection>
-[[Category:Topic Page]]
+[[Category: Topic Page]]