Suppose that you are carrying out studies of microtubule assembly via in vitro (in the test tube – not in the cell) assays. In a mixture of α-tubulin, β-tubulin, and abundant GTP, all suspended in a buffer that approximates cellular pH, you discover that you need the tubulin concentration to be at least 15 µM in order to observe any microtubule assembly. However, if you add centrosomes (including the proteins of the pericentriolar region) to your mixtures, you are able to observe microtubule assembly at concentrations of tubulin as low as 2 µM. Why does the addition of centrosomes to your mixture make such a difference in the concentration of tubulin needed to achieve assembly?
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The cell membrane is known by different names like plasma membrane or cytoplasmic membrane, or biological membrane. The term "cell membrane" was first introduced by C. Nageli and C. Cramer in the year 1855. Later on, in 1931, the term "plasmalemma" for cell membrane was given by J. Plowe. The cell membrane separates the cell's internal environment from the extracellular space. This separation allows the protection of cells from their environment.
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Suppose that you are carrying out studies of microtubule assembly via in vitro (in the test tube – not in the cell) assays. In a mixture of α-tubulin, β-tubulin, and abundant GTP, all suspended in a buffer that approximates cellular pH, you discover that you need the tubulin concentration to be at least 15 µM in order to observe any microtubule assembly. However, if you add centrosomes (including the proteins of the pericentriolar region) to your mixtures, you are able to observe microtubule assembly at concentrations of tubulin as low as 2 µM.
- Why does the addition of centrosomes to your mixture make such a difference in the concentration of tubulin needed to achieve assembly?
- At which end(s) of the microtubule would you expect addition of tubulin monomers to be occurring in your mixtures that include centrosomes?
- Suppose that you further altered your solution by replacing GTP with a GTP analogue that behaves like GTP in all respects, other than its ability to be hydrolyzed to produce GDP. (In other words, it’s essentially “stuck” as GTP). What change do predict that you’d observe in the overall pattern of
polymerization or depolymerization in your experiments (in comparison to those in which standard GTP was included)? Briefly explain your reasoning. Notes: (i) Part 3 is not a question about the concentration of tubulin necessary to achieve polymerization. (ii) The answer would be the same in the presence/absence of centrosomes.
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