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Proteins of the Myofibril

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Proteins of the Myofibril 131. PROTEINS OF THE MYUFlBRIL A. G. SZEWT-GYORGYI Ladies and gentlemen, I would like to review here a number of questions which involve some properties of the fibrous muscle proteins, how these properties lead to contractions, what the changes are which occur during contractions, and also to expand on the paper presented by Dr. Cassens by discussing how some of these proteins are responsible for the specialized structure of muscle and what reactions may control the forma- tion of these structures and activities. The muscle proteins can be conveniently divided into different fractions according to solubility and also according to function. If you grind up muscle and add a solvent of low ionic strength you will find that 35% of the proteins will be readily solubilized without, as I think will be shown on the first slide, really disrupting the specialized structure of the muscle. The striated pattern that apgeass in unextracted muscle, as shown by Dr. Cassens, will remain. What is solubilized is the enzymes of the glycolytic cycle, the phosphate-producing enzymes like myokinase and creetinekinase, and really the residue, that is, the insoluble portion remaining, will show the structural regularity of the muscle even clearer than before the extraction, We do not know exactly where these soluble enzymes are localized or if they are localized, but we think they are somewhere in the sarcoplasm and that they are not associated with the filamentous structure of muscle. I will not discuss these proteins any more. They do not concern us nor have anything to do with the culinary aspects of meat. Of course the relative munts of these soluble enzymes will greatly change depending on whether we are dealing with a red muscle or a white muscle. Now in muscle which was very well ground up, you will find that with the soluble proteins, a number of granules will be extracted. These granules compose the mitochondria and also the smoth endoplasmic reticulum, which is sometimes erroneously described as ribosomes, together with some actual ribosomes. Here, there will be a great deal mre interest for a person who is interested in how contraction is controlled, became the endoplasmic reticulum, which is this membranous system including the triad system with longitudinal components is closely connected with the problem of how stimulation travels inward to the muscles. One can, as Ebochet first did, isolate this relaxing or endoplasmic reticulum system and find that it acts as a relaxing system on the muscle. It indeed, even in an underrated condition, can act as a calcium pwp, concentrating calcium, and this activity will regulate the occurrence of coctraction. Finally there are about fifty or fifty-five per cent of the proteins which are not extractable with solvent of low ionic strength and which are responsible for the composition of myofibril and contraction. These proteins can be solubilized by using solvent of high ionic strength, e.g., 0.6 mlw mre salt, and include in case of mst muscles with the exception of annelid and mlusca muscles, mainly three proteins, the properties of which I will try to discuss in somewhat mre detail. The proteins remaining after extraction with high ionic strength solutions are called stroma proteins and consist mainly of collagen. The proportion of stoma protein in a muscle again will vary depending upon the type of muscle; I think it is in inverse relation- ship with the size of the muscle. We use rabbit psoas muscle for many ex- periments because of its lack of connective tissue, its softness, and the extreme ease of separating the fibers from each other. The next slide will show some of the properties and the distribu- tion of the main muscle groupings. Let's start with myosin, which is a protein of about 500,000 or 450,000 molecular weight, having a length of 1600 hgstroms. It is important to realize that the thick filaments are made up of myosin and that when discussing the banded structure of muscle, i.e., the A-band, the Z-band, and the I-band, that the length of this A-band is 1.6 microns. The individual myosin molecule is one-tenth of this length, or 0.16 microns, so there must be some type of aggregation of the qyosin molecules to make up the A-bands. The lqyosin mlecule has a number of properties. It has ATPase activity. It will combine with actin and the complex of actomyosin, which is formed from actin and myosin, shows the simple proper type of contrac- tion when ATP is added, provided the conditions are correct -- that is to say, at low ionic strength. Actomyosin has an unusual solubility property since it is precipitated at low ionic strength at which it contracts but it is dissolved at higher ionic strength. It is myosin which has the ATPase activity of the myofibrillar proteins, and what is mst important, contraction, individual, or for that matter, contraction of the nyofibril, occurs only under conditions where myosin is precipitated. This makes a headache if you hegpen to be a biochemist, because you cannot measure changes in length and shape during this contraction by using the tech- niques which were designed to study molecular solutions. Actually, perhaps the muscle proteins are one of those very few protein systems that we don't see and we can't readily measure any length and shape changes, al- though they contract visibly somehow. I will return a little bit later to myosin, but I would like to discuss now the properties of actin. Actin is again an unusual protein. It can exist in a monomeric form which is called globular actin, and which will polymerize under certain ideal conditions and form fibrous actin. The globular actin has a molecular weight of probably 60,000, and the F-actin has a molecular weight of several million. Actually, F-actin is a very regular aggregation of mnomeric particles, globular particles. It is an aggregation, but it forms a double helical chain, of the form shown by Dr. Cassens, and it has an almost indefinite length. That is why the molecular weight is indefinite although it is at least several millfon. What is very interesting is that the globular actin is associ- ated with ATP, but the fibrous actin is associated with ADP. Curing the polymerization process and only during this polymerization process, inorganic phosphate is liberated. This chemical change is closely asso- ciated with the structure or alteration of this molecule. 133. Now, there are some other interesting properties of actin. One of these is, as found by Gergely, that the ATP associated with G-actin is readily exchangeable with external ATP. Therefore, if ATP labeled with radioactive carbon is added to a solution of globular actin, the label will appear in the ATP bound to the actin. In the case of F-actin, the ADP does not exchange with ADP in solution, and if you add labeled ADP to a solution of F-actin, it will stay in the solution and not appear in the ADP bound to the F-actin. This is important and if I will have some time left, I will expound on it because it gives us a tool to study the state of the actin in muscle at rest or in contraction and to study whether any kind of change from one state to the other occurs during contraction. Now, let's discuss myosin further. msin is a fairly complex molecule, and under controlled conditions, treatment with protolytic enzymes will split it into two types of components. The next slide will show this reaction. Trypsin, chymtripsin or subtilisin will all give this type of splitting. The miyosin mlecule is split essentially into two types of components, called heavy meromyosin or light meromyosin. The next slide will show I think one of the early experiments involving the treatment of myosin with trypsin for varying lengths of time. The incubation periods in this experiment were two minutes, four minutes, six minutes, and twleve minutes with a one to two hundred trypsin-to-myosin ratio. You can see that myosin splits into slower and faster sedimenting components. The slower sedimenting component which can be crystallized is called light meromyosin and the faster sedhenting com- ponent is called heavy meroqyosin. At the time this experiment was con- ducted, we thought these results indicated that -sin is built up of at least two kinds of components which are attached end-to-end to each other. What is of great interest is that the heavier portion or heavy meromyosin has the center which is responsible for the ATPase activity of myosin, also possesses the center which combines with actin. The lighter component or light merorqyosin is responsible for the solubility properties of qyosin, that is, the precipitakion at low ionic strength and also the solution at high ionic strength. Heavy meromyosin is soluble regaxdless of the ionic strength. Also, light meromyosin is capable of forming extremely regular structures. If you measure the alpha-helix content of light meromyosin, you will find that light meromyosin fraction 1, which is about 25/35 of the total light meromyosin is almost lo@ alpha helical. It is one of the few proteins which behave as a fully called alpha-helix. If a solution of light meromyosin is precipitated by dilution and the precipitated protein examined directly under a microscope, a very regular 430x periodicity appears. This is one of the characteristic periods present in muscle and it is now explained as representing the myosin periodicity. Therefore, qyosin may be considered as having a head and a tail, and these expectations have been borne out very nicely in the electron microscope by a number of investigators starting from Rice and Hugh Huxley and Zobel and Carlson and so on.
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