Coordinate regulation of contractile protein synthesis during myoblast differentiation

The synthesis of contractile proteins has been studied during the differentiation of quail skeletal muscle myoblasts in culture. Myoblast differentiation was synchronized by transferring secondary cultures of rapidly dividing myoblasts into medium lacking cell division-promoting factors. Cultures at various stages of differentiation were then pulse-labeled with 35S-methionine, and cell extracts were resolved by electrophoresis on two-dimensional gels. Incorporation into specific proteins was quantitated by autoradiography and fluorography using a scanning densitometer. Contractile proteins synthesized by muscle cultures were identified by their co-electrophoresis on two-dimensional gels with contracile proteins purified from quail breast muscle. Our results show that the synthesis of myosin heavy chain, two myosin light chains, two subunits of troponin and two subunits of tropomyosin is first detected at the time of myoblast fusion and then rapidly increase at least 500 fold to maximum rates which remain constant in muscle fibers. Both the kinetics of activation and the molar rates of synthesis of these contractile proteins are virtually identical. Muscle-specific actin (alpha) synthesis also increases at the time of myoblast fusion, but this actin (alpha) is synthesized at 3 times the rate of other contractile proteins. The synthesis of 30 other muscle cell proteins was quantitated, and most of these are shown to follow different patterns of regulation. From these results, we conclude that the contractile proteins are regulated coordinately during myoblast differentiation.     

Turnover and shedding of Ia antigens by murine spleen cells in culture.

Lactoperoxidase-catalyzed radioiodination was used to examine the metabolic fate of surface Ia antigens on murine spleen cells in culture. Ia antigens, detected predominately on splenic B lymphocytes, were lost from the cultured cells with biphasic kinetics: a 4 to 6 hr rapid phase, t 1/2 = 5 hr followed by slow release through 20 hr, t 1/2 = 30 hr. The rapid loss of Ia antigens observed was abolished by both harsh iodination conditions and nonphysiologic incubation conditions. The rapid decline in Ia activity was shown to be due to shedding of intact Ia antigens from the cell and to predominant release of IA subregion-coded proteins. Release of Ia antigens from the cell was accomplished by replacement at the cell surface, and thus reflected net membrane Ia turnover. Ia shedding was shown to be extremely temperature dependent, reflecting both a comparatively high activation enthalpy and entropy requirement for turnover.     

AQUALINDERELLA FERMENTANS GEN. ET SP. NOV., A PHYCOMYCETE ADAPTED TO STAGNANT WATERS. I. MORPHOLOGY AND OCCURRENCE IN NATURE

Aqualinderella fermentans is described as a new genus and species in the order Leptomitales of the biflagellate, aquatic phycomycetes. Based upon pure as well as gross cultures, an account is given of its germination, its bipolar, unicellular, vegetative thallus, and its reproductive structures. The genus is distinguished particularly by an exceptionally coarse, digitately divided and thick-walled extramatrical body. The species is unique among known true fungi in that it combines a strictly fermentative metabolism with a requirement for high CO₂. It is specially adapted to grow saprophytically in warm, stagnant waters on submerged fruits and has been found in equatorial Africa, Central America, and the southern United States.