Myosin light chains regulatory chain

Smooth muscle myosin contains two myosin light chains. Phosphorylation of the regulatory light chain by myosin light chain kinase is a mandatory step to induce contraction. 

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

Rapid phosphorylation of the other detected phosphoproteins does occur but no definite roles have yet been ascribed to them. The 33 kDa protein may be the S6 ribosomal protein involved in the control of protein synthesis. The 57 kDa protein has been identified as the regulatory suhunit of the cyclic AMP-dependent protein kinase [44]. Of the other proteins the 76, 43 and 20 kDa may be connected with the microfilaments (43 kDa actin, 76 kDa myosin light chain kinase and 20 kDa myosin light chain) but this must be further investigated. These proteins may only play a permissive role in. steroidogenesis. The fact that the pattern of protein phosphorylation is very similar after stimulation of protein kinase C with phorbol esters supports this because the latter only marginally increase steroidogenesis [18]. 

Figure 34.5. Myosin Light Chains. The structures of the essential and regulatory light chains from muscle myosin are compared with the structure of calmodulin. Each of these homologous proteins binds an a helix (not shown) by wrapping around it.

Figure 9.7 Effects of cAMP contributing to relaxation. AC, adenylate cyclase DG, diacyl glycerol G, guanosine nucleotide regulatory protein IP3, Inositol trisphosphate, MLCK, myosin light chain kinase.

Chapter 1, this volume). Phosphorylation is also essential for movement of actin filaments in both the Nitella and the sliding actin in vitro motility assays (Umemoto and Sellers, 1990 Warshaw etai, 1990 Sellers etal., 1985). The site of the regulatory phosphorylation is Ser-19 on the LC20 (Pearson et al., 1984). Myosin light chain kinase (MLCK) also phosphorylates Thr-18, albeit at a much lower rate (Ikebe et al., 1986). Thr-18 phosphorylation increases the actin-activated MgATPase activity (Ikebe et al., 1988), but does not increase the rate of actin filament sliding in either of the two motility assays (Sellers et al., 1985 Okagaki et al, 1991). 

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