Teru Hayashi

Actin participation in actomyosin contraction

Abstract 1. 1. Pellicular fibers made of actin-free myosin show no ability to contract, at either pH 7.6 or 9.0. 2. 2. Fibers of actomyosin contract strongly and optimally at pH 7.6, but do not contract at pH 9.0. 3. 3. With increasing actin content, up to approximately 27%, the actomyosin fibers exhibit an increasing contractile activity. 4. 4. Fibers formed of actomyosin and of myosin hydrolyze ATP enzymically, the former at 7.6, the latter at pH 6.4 and 9.5. 5. 5. Myosin fibers, formed and maintained in the cold, do not contract when the temperature is raised in the presence of ATP, whereas actomyosin fibers under the same conditions, contract strongly with the rise in temperature. 6. 6. It is concluded that myosin without actin does not contract, and that actin is a specific requirement for contraction in the muscle protein model system.

The effects of colchicine on contractile proteins.

Abstract 1. 1. The possible mechanism of action of colchicine on dividing cells was tested by studying the effects of colchicine on the contractile proteins myosin, actomyosin, and actin. 2. 2. At a concentration of 1.0 mg/ml, colchicine had no inhibitory effect on the ATPase activity of actomyosin, either in solution in high salt medium, or in suspension in low salt medium. 3. 3. Likewise, colchicine had no effect on the ATPase activity of myosin in low salt medium, in contradiction to the findings of other workers. Some reasons for these differences in results are discussed. 4. 4. Colchicine inhibits the polymerization of actin. 5. 5. The relation of these findings to the studies of the mitotic spindle is examined.

A low viscosity G-actin preparation

Recently reported values for the intrinsic viscosity of G-ATP actin2 have been about 0.1 dl/g. (e.g., Lewis etal. (1963).). This value is considerably higher than one would expect for a globular protein. The possibility remains that these preparations contain small amounts (ca. 1%) of tropomyosin, which could make a considerable contribution to the intrinsic viscosity. We wish to report some preliminary physical measurements on G-ADP actin2 in the presence of 0.1 M salts. In addition to extracting the acetone powder at 0°C as suggested by Drabikowski and Gergely (1962), a further purification step to remove possible tropomyosin contamination was introduced.

Enzyme—substrate stabilization with surface-denaturated pepsin—albumin

Abstract 1. 1. Pepsin and albumin molecules will form autodigestible fibers from a compressed monomolecular film if they are mixed in solution prior to application to an air—water interface. 2. 2. When pepsin and albumin molecules in solution are placed separately on an air—water interface, the fibers formed are not autodigestible. 3. 3. Autodigestible fibers are formed from pepsin—albumin mixtures in the pH range of 4.4–6.6. When the proteins are mixed at pH 7.2, the fibers formed are not autodigestible. 4. 4. Curves obtained by precipitating these proteins with ammonium sulfate show that, although pepsin and albumin have distinctly different sensitivities to the action of this reagent, they precipitate out together when in mixture. 5. 5. The two proteins precipitate out together in the same pH range (4.4–6.6) as found for autodigestible fibers. At pH 7.2 they come down out of solution separately, according to their different solubilities. 6. 6. Mixtures containing albumin in excess show at pH 4.4 two separate precipitates. 7. 7. A tentative scheme is proposed to explain the data. According to this scheme, pepsin and albumin mixed at pH 4.4–6.6 combine to form an enzyme—substrate complex, which is stabilized when spread at an air—water interface and compressed to a fiber. The autodigestion of this fiber at pH 1.5 is taken to be the degradation of this stabilized complex.

The contraction of DNA-myosin fibers☆

Abstract Fibers formed of the DNA-myosin complex were tested for their ability to contract under the influence of ATP. At pH 7.6, no contraction was observed, in contrast to the actin-myosin fibers which contracted strongly. The DNA-myosin fibers were found to contract and relax reversibly with changes in pH, both in the presence of, and in the absence of ATP. Within the range tested (pH 7.6-4.0), the degree of contraction increased the more acid the medium, a measurable tension becoming apparent at about pH 5.6. It is concluded that the molecular basis of contraction of DNA-myosin is different from that of actin-myosin, and that DNA cannot substitute for actin in the mechanism of actomyosin contraction.