An electron microscope and X-ray study of actin: I. Electron microscope

Abstract

1. 1. Electron microscope studies have been made of F-actin, prepared by activating G-actin by the addition of KCl. It is fibrous and the fibres appear to form by the joining-together of corpuscular units. The results confirm in general the findings of Jakus and Hall and support the indirect conclusions of Straub and Szent-Györgyi. 2. 2. G-actin is found to consist of corpuscular bodies. 3. 3. Actomyosin, prepared from actin and myosin in physiological proportions, is found to surpass either of its components in the capacity of forming anastomosed fibrous networks, but the property is lost in the presence of ATP. 4. 4. The interpretation of these observations is discussed. 5. 5. An X-ray fibre photograph of F-actin has been obtained. The fibre period is at least 54 A (approx.), but it may possibly be twice this. 6. 6. The analogy between the fibre diagrams of F-actin and feather keratin is pointed out, and it is suggested that in the fundamental scheme of the keratin-myosin-epidermis-fibrinogen group, F-actin may be related to myosin as feather keratin is to mammalian α-keratin. 7. 7. The F-actin patterns is discussed in relation to previous indications that protein fibres may originate in the end-to-end addition of initially corpuscular units. 8. 8. The detailed construction of the F-actin fibre diagram indicates that the corpuscular units from which the fibres are formed are not strung together in arbitrary fashion, but always in the same way and with atomic precision. 9. 9. Actomyosin is found to give a large-angle X-ray pattern similar to that of myosin. 10. 10. F-actin passes into the normal β-configuration on heating to 60° C.