Caldesmon structure

In the smooth muscle cell, CD is incorporated into the thin filaments in the "contractile domain" of the cell (Furst et al., 1986 North et al., 1994a). Ultrastruc-tural studies presented in Chapter 4 (this volume) have shown that CD is located in the thin filament in an extended form beside TM along the axis of the actin double helix. The model (Fig. 2) places CD in potential contact with actin and TM throughout its length and allows a possible end-to-end interaction. These structural arrangements form the basis of caldesmon function in the thin filament. 

In vitro experimentation has shown that caldesmon is an integral component of smooth muscle thin filaments and plays a central role in their Ca2+-dependent regulation. A mechanism analogous to that of troponin has been proposed this now requires extensive testing. The structure of the regulatory domain of caldesmon is not well defined and we look forward to being able to describe this in three dimensions and in combination with its physiological partners actin, tropomyosin, and calmodulin. 

These two phenotypes of smooth muscle, in addition to having marked differences in contractile activity, express different isoforms of several contractile proteins and certain soluble enzymes. In particular, proliferative smooth muscle contains at least three PDPKs p34 i 2 p42 , and p44 PK the contractile phenotype of smooth muscle, only p42 P and p44 mark have been identified. The precise function and a complete description of the substrates for MAPK in the contractile phenotype of smooth muscle are unknown however, one substrate that has been idenhfied is the actin and myosin binding protein, caldesmon. Because of the phosphorylation of cal-desmon, MAPK may be involved in either smooth muscle contractile regulation or the structural organization of actin filaments within smooth muscle cells. 

FIGURE 4 Schematic depicting the potential roles for MAPK in contractile versus proliferative (or cultured) smooth muscle. MAPK is activated in response to stimulation by growth factors, stretch, and pharmacological agents. This process can be inhibited by cAMP and cAMP-dependent protein kinase. Once activated, MAPK phos-phorylates a number of intracellular proteins (both cytoplasmic and nuclear) that result in an alteration of growth and proliferation in cultured cells. In contractile smooth muscle, MAPK phosphorylation of caldesmon may lead to alterations in muscle contractility or actin filament structure. 

Frank ED, Warren L (1981) Aortic smooth muscle cells contain vimentin instead of desmin. Proc Natl Acad Sci U S A 78 30203024 Fuglsang A, Khromov A, Torok K, Somlyo AV, Somlyo AP (1993) Flash photolysis studies of relaxation and cross-bridge detachment higher sensitivity of tonic than phasic smooth muscle to MgADP. J Muscle Res Cell Motil 14 6677 Furst DO, Cross RA, De Mey J, Small JV (1986) Caldesmon is an elongated, flexible molecule localized in the actomyosin domains of smooth muscle. EMBO J 5 251257 Gabella G (1984) Structural apparatus for force transmission in smooth muscles. Physiol Rev 64 455477... 

Lehman W, Craig R, Lui J, Moody C (1989) Caldesmon and the structure of smooth muscle thin filaments immunolocalization of caldesmon on thin filaments. J Muscle Res Cell Motil 10 101112... 

Maniotis AJ, Chen CS, Ingber DE (1997) Demonstration of mechanical connections between integrins, cytoskeletal filaments, and nucleoplasm that stabilize nuclear structure. Proc Natl Acad Sci U S A 94 849854 Marston S (1990) Stoichiometry and stability of caldesmon in native thin filaments from sheep aorta smooth muscle. Biochem J 272 305310 Marston S, Pinter K, Bennett P (1992) Caldesmon binds to smooth muscle myosin and myosin rod and crosslinks thick filaments to actin filaments. J Muscle Res Cell Motil 13 206218... 

Structural data have suggested that tropomyosin can alter its position on smooth muscle thin filaments following cross-bridge or caldesmon binding (Vibert et al. 1972,1993, Arner et al. 1988). The structural effects of caldesmon on tropomyosin are however not identical to those induced by troponin in striated muscle thin filaments (Hodgkinson et al. 1997, Lehman et al. 1997). Biochemical data show that smooth muscle thin... 

Vibert P, Craig R, Lehman W (1993) Three-dimensional reconstruction of caldesmon-containing smooth muscle thin filaments. J Cell Biol 123 313-321 Vibert PJ, Haselgrove JC, Lowy J, Poulsen FR (1972) Structural changes in actin-con-taining filaments of muscle. J Mol Biol 71 757-767 Vorotnikov AV, Gusev NB, Hua S, Collins JH, Redwood CS, Marston SB (1993) Identification of casein kinase II as a major endogeneous caldesmon kinase in sheep aorta smooth muscle. FEBS Lett 334 18-22... 

Both proteins exist as structural variants hi (equivalent to chicken a-isoform) and I calponin (homologous to chicken P-isoform),and h- and Z-caldesmon [124]. A new type of calponin (h2), containing 57 amino acids at the COOH-terminus with a strong acidic domain, is also expressed in adult rat SM and NM tissues/cells [125]. The hi calponin and Zi-caldesmon are mainly or exclusively found in SMC [126,127], whilst the Z-caldesmon is widely expressed in NM tissues/cells. The molecular weight deduced from the respective cDNA for Zi-caldesmon and Z-caldesmon is 87-93 kDa and 59-63 kDa, respectively. The two isoforms of caldesmon have identical sequences except for the insertion of the central repeating domain in Zi-caldesmon. 

Numerous laboratories have observed that the cell structure as well as cytoskeletal and cytocontractile protein content is markedly different between the two opposite SMC phenotypes. In the synthetic phenotype SM2, and to a lesser extent SMI, SM-type a-actin, h-caldesmon, calponin and desmin content/distribution are decreased, whereas NM-type MyHC-... 

Hayashi K, Fujio Y, Kato I, Sobue K (1991) Structural and functional relationships between h- and /-caldesmons. J Biol Chem 266 355-361... 

Hayashi K, Yano K, Hashida T, Takeuchi R, TakedaO, Asada K, Takahashi E, Kato I, Sobue K (1992) Genomic structure of human caldesmon gene. Proc Natl Acad Sci USA 89 12122-12126... 

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