Myosin light-chain kinase explained

Myosin Light-Chain kinase, smooth muscle
Hgncid:	7590
Symbol:	MYLK
Entrezgene:	4638
Omim:	600922
Refseq:	NM_053025
Uniprot:	Q15746
Ecnumber:	2.7.11.18
Chromosome:	3
Arm:	q
Band:	cen
Locussupplementarydata:	-q21

myosin light-chain kinase 2, skeletal muscle
Caption:	Crystal structure of the S. dubia centrin / human skeletal muscle myosin light-chain complex.^[1]
Hgncid:	16243
Symbol:	MYLK2
Entrezgene:	85366
Omim:	606566
Refseq:	NM_033118
Uniprot:	Q9H1R3
Chromosome:	20
Arm:	q
Band:	13.31

myosin light-chain kinase 3, cardiac
Hgncid:	29826
Symbol:	MYLK3
Entrezgene:	91807
Omim:	612147
Refseq:	NM_182493
Uniprot:	Q32MK0
Chromosome:	16
Arm:	q
Band:	11.2

Human Myosin Light-Chain Kinase
Caption:	The Crystal Structure of the Human Myosin Light Chain Kinase Loc340156.^[2]
Hgncid:	27972
Symbol:	MYLK4
Entrezgene:	340156
Refseq:	NM_001012418
Uniprot:	Q86YV6
Arm:	6
Band:	p
Locussupplementarydata:	25.2

Myosin light-chain kinase also known as MYLK or MLCK is a serine/threonine-specific protein kinase that phosphorylates a specific myosin light chain, namely, the regulatory light chain of myosin II.^[3]

General structural features

While there are numerous differing domains depending on the cell type, there are several characteristic domains common amongst all MYLK isoforms. MYLK’s contain a catalytic core domain with an ATP binding domain. On either sides of the catalytic core sit calcium ion/calmodulin binding sites. Binding of calcium ion to this domain increases the affinity of MYLK binding to myosin light chain. This myosin binding domain is located at the C-Terminus end of the kinase. On the other side of the kinase at the N-Terminus end, sits the actin-binding domain, which allows MYLK to form interactions with actin filaments, keeping it in place.^{[4] [5]}

Isoforms

Four different MYLK isoforms exist:^[6]

• MYLK1 – smooth muscle
• MYLK2 – skeletal
• MYLK3 – cardiac
• MYLK4 – novel

Function

These enzymes are important in the mechanism of contraction in muscle. Once there is an influx of calcium cations (Ca²⁺) into the muscle, either from the sarcoplasmic reticulum or from the extracellular space, contraction of smooth muscle fibres may begin. First, the calcium will bind to calmodulin.^[7] After the influx of calcium ions and the binding to calmodulin, pp60 SRC (a protein kinase) causes a conformational change in MYLK, activating it and resulting in an increase in phosphorylation of myosin light chain at serine residue 19. The phosphorylation of MLC will enable the myosin crossbridge to bind to the actin filament and allow contraction to begin (through the crossbridge cycle). Since smooth muscle does not contain a troponin complex, as striated muscle does, this mechanism is the main pathway for regulating smooth muscle contraction. Reducing intracellular calcium concentration inactivates MLCK but does not stop smooth muscle contraction since the myosin light chain has been physically modified through phosphorylation(and not via ATPase activity). To stop smooth muscle contraction this change needs to be reversed. Dephosphorylation of the myosin light chain (and subsequent termination of muscle contraction) occurs through activity of a second enzyme known as myosin light-chain phosphatase (MLCP).^[8]

Upstream Regulators

Protein kinase C and ROC Kinase are involved in regulating Calcium ion intake; these Calcium ions, in turn stimulate a MYLK, forcing a contraction.^[9] Rho kinase also modulates the activity of MYLK by downregulating the activity of MYLK's counterpart protein: Myosin Light Chain Phosphatase (MYLP).^[10] In addition to downregulation of MYLK, ROCK indirectly strengthens actin/myosin contraction through inhibiting Cofilin, a protein which depolymerizes actin stress fibers.^[11] Similar to ROCK, Protein Kinase C regulates MYLK via the CPI-17 protein, which downregulates MYLP.^[12]

Mutations and resulting diseases

Some pulmonary disorders have been found to arise due to an inability of MYLK to function properly in lung cells. Over-activity in MYLK creates an imbalance in mechanical forces between adjacent endothelial and lung tissue cells. An imbalance may result in acute respiratory distress syndrome, in which fluid is able to pass into the alveoli.^[13] Within the cells, MYLK provides an inward pulling force, phosphorylating myosin light chain causing a contraction of the myosin/actin stress fiber complex. Conversely, cell-cell adhesion via tight and adherens junctions, along with anchoring to extra cellular matrix (ECM) via integrins and focal adhesion proteins results in an outward pulling force. Myosin light chain pulls the actin stress fiber attached to the cadherin, resisting the force of the adjacent cell's cadherin. However, when the inward pulling force of the actin stress fiber becomes greater than the outward pulling force of the cell adhesion molecules due to an overactive MYLK, tissues can become slightly pulled apart and leaky, leading to passage of fluid into the lungs.^[14]

Another source of smooth muscle disorders like ischemia–reperfusion, hypertension, and coronary artery disease arise when mutations to protein kinase C (PKC) result in excessive inhibition of MYLP, which counteracts the activity of MYLK by dephosphorylating myosin light chain. Because myosin light chain has no inherent phosphate cleaving property over active PKC prevents the dephosphorylation of myosin light protein leaving it in the activated conformation, causing an increase in smooth muscle contraction.

See also

• protein kinase A

Further reading

• Clayburgh DR, Rosen S, Witkowski ED, Wang F, Blair S, Dudek S, Garcia JG, Alverdy JC, Turner JR . A differentiation-dependent splice variant of myosin light chain kinase, MLCK1, regulates epithelial tight junction permeability . . 279 . 53 . 55506–13 . December 2004 . 15507455 . 1237105 . 10.1074/jbc.M408822200 . free .
• Wang F, Graham WV, Wang Y, Witkowski ED, Schwarz BT, Turner JR . Interferon-gamma and tumor necrosis factor-alpha synergize to induce intestinal epithelial barrier dysfunction by up-regulating myosin light chain kinase expression . . 166 . 2 . 409–19 . February 2005 . 15681825 . 1237049 . 10.1016/S0002-9440(10)62264-X .
• Russo JM, Florian P, Shen L, Graham WV, Tretiakova MS, Gitter AH, Mrsny RJ, Turner JR . Distinct temporal-spatial roles for rho kinase and myosin light chain kinase in epithelial purse-string wound closure . Gastroenterology . 128 . 4 . 987–1001 . April 2005 . 15825080 . 1237051 . 10.1053/j.gastro.2005.01.004 .
• Shimizu S, Yoshida T, Wakamori M, Ishii M, Okada T, Takahashi M, Seto M, Sakurada K, Kiuchi Y, Mori Y . Ca2+-calmodulin-dependent myosin light chain kinase is essential for activation of TRPC5 channels expressed in HEK293 cells . . 570 . Pt 2 . 219–35 . January 2006 . 16284075 . 1464317 . 10.1113/jphysiol.2005.097998 .
• Kim MT, Kim BJ, Lee JH, Kwon SC, Yeon DS, Yang DK, So I, Kim KW . Involvement of calmodulin and myosin light chain kinase in activation of mTRPC5 expressed in HEK cells . American Journal of Physiology. Cell Physiology . 290 . 4 . 1031–40 . April 2006 . 16306123 . 10.1152/ajpcell.00602.2004 .
• Connell LE, Helfman DM . Myosin light chain kinase plays a role in the regulation of epithelial cell survival . . 119 . Pt 11 . 2269–81 . June 2006 . 16723733 . 10.1242/jcs.02926 . 19038438 . free .
• Seguchi O, Takashima S, Yamazaki S, Asakura M, Asano Y, Shintani Y, Wakeno M, Minamino T, Kondo H, Furukawa H, Nakamaru K, Naito A, Takahashi T, Ohtsuka T, Kawakami K, Isomura T, Kitamura S, Tomoike H, Mochizuki N, Kitakaze M . A cardiac myosin light chain kinase regulates sarcomere assembly in the vertebrate heart . . 117 . 10 . 2812–24 . October 2007 . 17885681 . 1978424 . 10.1172/JCI30804 .
• Hong F, Haldeman BD, Jackson D, Carter M, Baker JE, Cremo CR . 15 June 2011. Biochemistry of Smooth Muscle Myosin Light Chain Kinase. . 510. 2. 135–146. 10.1016/j.abb.2011.04.018. 0003-9861. 3382066. 21565153.

Notes and References

1. RCSB Protein Data Bank - Structure Summary for 3KF9 - Crystal structure of the SdCen/skMLCK complex . Worldwide Protein Data Bank . 10.2210/pdb3kf9/pdb . 2011 . Radu . L. . Assairi . L. . Blouquit . Y. . Durand . D. . Miron . S. . Charbonnier . J.B. . Craescu . C.T. .
2. RCSB Protein Data Bank - Structure Summary for 2X4F - The Crystal Structure of the Human Myosin Light Chain Kinase Loc340156. . Worldwide Protein Data Bank . 10.2210/pdb2x4f/pdb . 2010 . Muniz . J.R.C. . Mahajan . P. . Rellos . P. . Fedorov . O. . Shrestha . B. . Wang . J. . Elkins . J.M. . Daga . N. . Cocking . R. . Chaikuad . A. . Krojer . T. . Ugochukwu . E. . Yue . W. . von Delft . F. . Arrowsmith . C.H. . Edwards . A.M. . Weigelt . J. . Bountra . C. . Gileadi . O. . Knapp . S. .
3. Gao Y, Ye LH, Kishi H, Okagaki T, Samizo K, Nakamura A, Kohama K . Myosin light chain kinase as a multifunctional regulatory protein of smooth muscle contraction . IUBMB Life . 51 . 6 . 337–44 . June 2001 . 11758800 . 10.1080/152165401753366087 . 46180993 .
4. Khapchaev AY, Shirinsky VP . Myosin Light Chain Kinase MYLK1: Anatomy, Interactions, Functions, and Regulation . Biochemistry. Biokhimiia . 81 . 13 . 1676–1697 . December 2016 . 28260490 . 10.1134/S000629791613006X . 11424747 .
5. Stull JT, Lin PJ, Krueger JK, Trewhella J, Zhi G . Myosin Light Chain Kinase: Functional Domains and Structural Motifs . Acta Physiologica . 164 . 4 . 471–482 . December 1998 . 10.1111/j.1365-201X.1998.tb10699.x. 9887970 . free .
6. Manning G, Whyte DB, Martinez R, Hunter T, Sudarsanam S . The protein kinase complement of the human genome . Science . 298 . 5600 . 1912–34 . December 2002 . 12471243 . 10.1126/science.1075762 . 2002Sci...298.1912M . 26554314 .
7. Book: Robinson . A . Colbran . R . Lennarz . William . Lane . Daniel . vanc . Calcium/Calmodulin-Dependent Protein Kinases . Encyclopedia of Biological Chemistry . Elsevier inc. . 2nd . 304–309 . 2013 . 978-0-12-378631-9 .
8. Book: Feher, Joseph . vanc . Smooth Muscle . Quantitative Human Physiology . 2nd . Elsevier inc. . 2017 . 351–361 . 978-0-12-800883-6.
9. Anjum I . Calcium sensitization mechanisms in detrusor smooth muscles . Journal of Basic and Clinical Physiology and Pharmacology . 29 . 3 . 227–235 . January 2018 . 29306925 . 10.1515/jbcpp-2017-0071 . 20486807 .
10. Amano M, Nakayama M, Kaibuchi K . Rho-kinase/ROCK: A key regulator of the cytoskeleton and cell polarity . Cytoskeleton . 67 . 9 . 545–54 . September 2010 . 20803696 . 3038199 . 10.1002/cm.20472 .
11. Dudek SM, Garcia JG . Cytoskeletal regulation of pulmonary vascular permeability . Journal of Applied Physiology . 91 . 4 . 1487–500 . October 2001 . 11568129 . 10.1152/jappl.2001.91.4.1487 . 7042112 .
12. Book: Ringvold HC, Khalil RA . Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders . 78 . 203–301 . 2017 . 28212798 . 5319769 . 10.1016/bs.apha.2016.06.002 . 978-0-12-811485-8 . Advances in Pharmacology . Vascular Pharmacology - Smooth Muscle .
13. Szilágyi KL, Liu C, Zhang X, Wang T, Fortman JD, Zhang W, Garcia JG . Epigenetic contribution of the myosin light chain kinase gene to the risk for acute respiratory distress syndrome . Translational Research . 180 . 12–21 . February 2017 . 27543902 . 5253100 . 10.1016/j.trsl.2016.07.020 .
14. Cunningham KE, Turner JR . Myosin Light Chain Kinase: Pulling the Strings of Epithelial Tight Junction Function . Annals of the New York Academy of Sciences . 1258 . 1 . 34–42 . July 2012 . 10.1111/j.1749-6632.2012.06526.x. 22731713 . 3384706 . 2012NYASA1258...34C .