Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 14

4 Activation ("maturation") of frog oocytes is sig- naled through a MAP kinase signaling module. An increase in the hormone progesterone triggers the module by stim- ulating the translation of Mos mRNA, which is the frog's MAP kinase kinase kinase (Figure Q15-2). Maturation is easy to score visually by the presence of a white spot in the middle of the brown surface of the oocyte (see Figure Q15-2). To determine the dose-response curve for pro- gesterone-induced activation of MAP kinase, you place 16 oocytes in each of six plastic dishes and add various con- centrations of progesterone. After an overnight incubation, you crush the oocytes, prepare an extract, and determine the state of MAP kinase phosphorylation (hence, activa- tion) by SDS polyacrylamide-gel electrophoresis (Figure Q15-3A). This analysis shows a graded response of MAP kinase to increasing concentrations of progesterone. Before you crushed the oocytes, you noticed that not all oocytes in individual dishes had white spots. Had some oocytes undergone partial activation and not yet reached the white-spot stage? To answer this question, you repeat the experiment, but this time you analyze MAP kinase acti- vation in individual oocytes. You are surprised to find that each oocyte has either a fully activated or a completely inactive MAP kinase (Figure Q15-3B). How can an all-or- none response in individual oocytes give rise to a graded response in the population?