Skeletal muscle regulatory proteins

Adapted from Ohtsuki, L, Maruyama, K., and Eba.shi, S., 1986. Regulatory and cyto.skeletal protein.s of vertebrate. skeletal muscle. Advances in Protein Chemistry 38 1-67. 

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

Figure 8.11 Aggregation of F-actin, tropomyosin and troponin to form the thin filaments of myofibrils. (Reproduced by permission from Ohtsuki I, Maruyama K, Ebashi S. Regulatory and cytoskeletal protein of vertebrate skeletal muscle. Adv Prot Chem 38 1— 60, 1986.)...

Two key regulatory enzymes involved in the control of glycogen metabolism were first recognized as targets of cAMP and cAMP-dependent protein kinase in liver and skeletal muscle. These are phosphorylase b kinase and glycogen synthase. The molecular details of the phosphorylation and regulation of these enzymes are better understood in muscle than in liver since the liver enzymes have only recently been purified to homogeneity in the native form. However, it appears that they share many key features in common. 

The contractile proteins of the myofibril include three troponin regulatory proteins. The troponin complex includes three protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component). The subunits exist in a number of isoforms. The distribution of these isoforms varies between cardiac muscle and slow- and fast-twitch skeletal muscle. Only two major isoforms of troponin C are found in human heart and skeletal muscle. These are characteristic of slow- and fast-twitch skeletal muscle. The heart isoform is identical with the slow-twitch skeletal muscle isoform. Isoforms of cardiac-specific troponin T (cTnT) and cTnl also have been identified and are the products of unique genes. All cardiac troponins are localized primarily in the myofibrils (94%-97%), with a smaller cytoplasm fraction (3%-6%). 

The contraction of ascidian smooth muscle was found to be regulated through the troponin-tropomyosin system. But the action of troponin components was different from that of troponin of vertebrate striated muscles (Endo and Obinata, 1981). In this system, the inhibitory action of troponin I (MW 24,000) is less remarkable compared with vertebrate skeletal troponin I, and troponin C (MW 18,000) does not neutralize the inhibition by troponin I. But upon further addition of troponin T (MW 33,000) in the concomitant presence of all three components and tropomyosin, the contractile interaction of myosin and actin is activated. In this case, the action of troponin T has some similarity with that of the above-mentioned cardiac troponin T hybridized with skeletal troponin C-I. Since actomyosin, without these regulatory proteins, is inhibited regardless of Ca concentration, Ca " and troponin-tropomyosin are activators for contraction of actomyosin in ascidian smooth muscle. In this respect, the type of Ca + regulation of ascidian smooth muscle is the same as that for vertebrate smooth muscles which do not contain troponin (Ebashi, 1980). 

In this section, properties of calcium regulatory proteins from vertebrate skeletal muscle were reviewed with particular reference to the physiological structure and function that have been the major interest since the discovery of native tropomyosin. 

Regulatory and Cytoskeletal Proteins of Vertebrate Skeletal Muscle... 

Protein phosphatases-1, 2 A, 2B, and 2C occur in mammalian liver and, as in skeletal muscle, possess essentially all of the phosphatase activity toward enzymes and regulatory proteins of glycogen metabolism. In liver, however, the ratios of the activities of phosphatase-2A and 2C to that of phosphatase-1 are seven-fold higher than in muscle. Although protein phosphatase-I sediments with glycogen particles in both tissues, a much smaller fraction is glycogen-associated in liver than in muscle. The specific activity of phosphatase-2B is lower in liver than in muscle. Protein phosphatase inhibitors-1 and 2 have been identified in liver, where they appear to function as they do in muscle. A disinhibitor protein (M. W. 9,000) of liver can block the effects of inhibitors-1 and 2 on phosphatase-1. 

Another very long protein (nebulin) is associated with the thin filaments. Nebulin has a molecular weight of about 700,000. An abundant protein in skeletal muscle, nebulin extends from either side of the Z-disks along the entire length of the thin filaments. It may serve as a template for thin-filament assembly, and may interact with tropomyosin, and also may have a regulatory role. 

Similarly, multiple genes, alternative RNA splicing, and posttranslational modifications result in multiple essential and regulatory light chains, tropomyosins, titins, and other myofibrillar proteins. Energy pathway enzymes are differentially expressed in various skeletal fiber types, in cardiac and smooth muscle, and at different stages of development. This also applies to Ca regulatory proteins such as the... 

Phosphorylase kinase is the enzyme that activates phosphorylase b by attaching a phosphoryl group. Its subunit composition in skeletal muscle is (apy8)4, and the mass of this very large protein is 1200 kd. The catalytic activity resides in the y subunit, whereas the other subunits serve regulatory functions. This kinase is under dual control it is activated both by phos-... 

The catalytic subunit of PPl is a 37 kd single-domain protein. This subunit is usually bound to one of a family of regulatory subunits with masses ol approximately 120 kd in skeletal muscle and heart, the most prevalent regulatory subunit is called Gvr. whereas, in the liver, the most prevalent subunit is Gl. These regulatory subunits have modular structures with domains that participate in interactions with glycogen, with the catalytic subunit, and... 

Tallini G, Parham DM, Dias P, et al. Myogenic regulatory protein expression in adult soft tissue sarcomas A sensitive and specific marker of skeletal muscle differentiation. Am J Pathol. 1994 144 693-701. 

Availability of antibodies to myogenic transcription factors myogenin (also called myf-4) and Myo-Dl has greatly improved the ability to demonstrate a skeletal muscle phenotype in suspected RMS. Myogenin and Myo-Dl are myogenic transcriptional regulatory proteins... 

---