Myosin inhibitors

Cytokinesis (cell division) in animal cells involves the progressive formation in telophase of a furrow between the two daughter cells in the equator of the mitotic spindle. Immunofluorescent staining of the cortical cytoplasm at the site of the contraction ring reveals an abundance of actin as well as myosin, a-actinin, and filamin (Fishkind and Wang, 1995). Cytokinesis is highly sensitive to actin-myosin inhibitors such as cytochalasin and phalloidin. 

As noted previously, myosin is located both in the lamellipodium and uropod of polarized cells [91, 401]. Whereas the myosin inhibitor, 2,3-butanedione monoxime, inhibits lamellapodial extension at high concentrations, at intermediate concentrations cell polarization and lamellipodium extension occur, but uropod retraction is inhibited, clearly indicating a role for myosin-mediated contractile forces in uropod retraction [91, 398]. The inhibitor, 2,3-butanedione monoxime, inhibits multiple forms of myosin [72], thus lamellapodial extension and uropod retraction may be mediated by different forms of myosin. 

Urwyler, N., Eggli. P. and KeUer, H.U. (2000). Effects of the myosin inhibitor 2,3-butanedione monoxime (BDM) on ceU shape, locomotion and fluid pinocytosis in human polymorphonuclear leukocytes. Cell Biol. Inti. 24, 863-870. 

Kepiro M, Vaikuti BH, Bodor A, Hegyi G, Drahos L, Kovacs M, Malnasi-Csizmadia A. Azidoblebbistatin, a photraeactive myosin inhibitor. Proc Natl Acad Sci USA. 2112 109 9402-9407. 

If MLCK activates contraction by increasing myosin phosphorylation, then an increase in the activity of myosin light chain phosphatase, MLCP, by decreasing the fraction of myosin which is phosphorylated, should lead to relaxation from the active (contractile) state. Cyclic adenosine monophosphate (AMP) is a strong inhibitor of smooth muscle contraction and it has been suggested that activation of MLCP could result from its phosphorylation via cAMP activated protein kinase (see Figure 5). 

Inhibitor of F-actin-myosin interaction (inhibitor of F-actin-dependent activation of ATPase) Troponin system (Tpl) Unphosphorylated myosin light chain... 

Fig. 2.1. Semilogarithmic plot of molecular weight (Mr) of marker proteins vs relative mobility (Rf) of marker proteins in gels of different acrylamide concentrations %T. Proteins 1 aprotinin (6.5 kD) 2 lysozyme (14.5 kD) 3 soybean trypsin inhibitor (21.5 kD) 4 carbonic acid anhydrase (31 kD) 5 hen ovalbumin (45 kD) 6 bovine serum albumin (66 kD) 7 phosphorylase b (97.4 kD) 8 8-galactosidase (116 kD) 9 myosin (205 kD)...

Nakanishi, S. Kakita, S. Takahashi, I. Kawahara, K. Tsukada, E. Sano, T. Yamada, K. Yoshida, M. Kase, H. Matsuda, Y. Hashimoto, Y. Nonomura, Y. Wortmannin, a microbial product inhibitor of myosin light chain kinase. J. Biol. Chem., 267, 2157-2163 (1992)... 

Nakanishi, S. Ando, K. Kawamoto, I. Matsuda, Y. MS-347a, a new inhibitor of myosin light chain kinase from Aspergillus sp. KY52178. J. Antibiot., 46, 1775-1781 (1989)... 

Keratin IFs exhibit different motile behaviors compared with type III and IV IFs, and continue to move in the absence of MT in cells treated with MT inhibitors, such as nocodazole. It has been proposed that actin may be involved in their transport (Yoon et al., 2001). Time lapse imaging of keratin-GFP has revealed that keratin IF formation begins in the cell cortex in a region enriched in actin and required for both intact MT and actin, and that actin governs the organization and movement of keratin in Xenopus egg extracts (Weber and Bement, 2002). Less is known about specific mediators of these interactions, although the fact that myosin Va associates with NFs raises the interesting possibility that other such interactions may exist for keratins. 

Based on this, one would predict that treatment of intact smooth muscle preparations with an inhibitor of CaMKII should potentiate myosin light chain phosphorylation and the rapid phase of force development. In contrast, we have found that in carotid arterial smooth muscle, KN-93 inhibition of CaMKII activation in response to physiological contractile stimuli correlates with a marked inhibition of tonic force responses (Rokolya and Singer 2000), suggesting an alternative dominant action of CaMKII on the smooth muscle contractile apparatus. 

Twelve ACE-inhibitory peptides have been identified from sardine muscle hydrolysate, revealing that a dipeptide, Val-Tyr, acts as a key inhibitor (Matsufuji et al., 1994). Of the identified ACE-inhibitory peptides, the tripeptides (Leu-Arg-Pro, lie-Val-Tyr) and the dipeptide (Val-Tyr) show strong inhibitory activity. Moreover, two inhibitory peptides (myopentapep-tides A and B) have been purified from a thermolysin digest of porcine skeletal muscle proteins. The sequences were found in the primary structure of the myosin heavy chain (Arihara et al., 2001). 

Apart from the phosphorylation theory, other regulatory mechanisms have also been suggested for smooth muscle contraction. A thin-filament protein that has been proposed as a regulatory component is caldesmon [102], Purified caldesmon is a potent inhibitor of actin-tropomyosin interaction with myosin. The mechanisms by which calcium removes this inhibition are controversial. Furthermore, phosphorylation of caldesmon by a caldesmon kinase in vitro has also been implicated in this... 

The final mitosis stage involves separation of two sets of chromosomes via microtubules that are filamentous polymers of tubulin monomers. Compounds that interfere with tubulin polymerization such as the plant-derived compounds colchicine, taxol, vinblastine and vincristine are cell division inhibitors (Table 9.6). The cytokinesis of the daughter cells requires equal division of cytoplasm and an actin-myosin-based contractile ring provides the force to make this separation. Accordingly, compounds such as cytochalasin B that interfere with actin will also interfere with cell division (Table 9.6). 

A. F. Straight, A small-molecule inhibitor of skeletal muscle myosin II. Nat. Cell Bio. 2002, 4, 83-84. 

T. J. Mitchison, Dissecting temporal and spatial control of cytokines with a myosin II inhibitor. Science 2003, 299, 1743-1747. 

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