You are studying chemotaxis in two different strains of Dictyostelium discoidum.

Question

You are studying chemotaxis in two different strains of Dictyostelium discoidum. The first strain has a mutation in the solo gene (solo mutant), which encodes the G subunit of the heterotrimeric G-protein complex that is coupled to the cAMP receptor. The resulting protein product has normal GTPase activity, but it is unable to bind downstream targets. The second strain has a mutation in the clumpy gene (clumpy mutant), which encodes the G subunit of the same heterotrimeric G protein complex. However, the resulting mutation prevents G from interacting with G. This causes constitutively active G.

A) A single cell from each strain is placed on dish near a needle that flows cAMP. The cAMP that flows from the needle establishes a gradient with the highest concentration at the needle tip. Briefly describe how each mutant strain will respond to the cAMP gradient. Justify your answer in 1-2 sentences.

i) solo mutant strain:

ii) clumpy mutant strain:

B) A population of cells is evenly distributed onto a dish and starved. Under these conditions, wild-type D. discoidum will produce a short pulse of cAMP (via a pathway that is unaffected in either mutant strain). Wild- type ameoba will then aggregate to form a fruiting body. Predict how a population of cells from each strain will respond to starvation. Will a fruiting body form? Justify your answer in 1-2 sentences.

i) solo mutant strain:

ii) clumpy mutant strain:

Please Help ASAP!

Explanation / Answer

Molecular genetics and biochemical studies of Dictyostelium gave us an understanding of the mechanisms of chemotaxis. Through chemotaxis, organism exhibit an attraction in response to chemoattractant and will become polarized in the direction of the chemoattractant and will form a leading edge or will orient their polarity in the direction of the chemoattractant gradient.

(i)In case of solo mutants, it can encode only G subunit of heterotrimeric G-protein complex and can couple to the cAMP receptor. And can exhibit normal GTPase activity. But cannot bind to downstream targets.

But the response towards chemoattractant need the activation PI3 kinase and downstream effectors Akt/PKB. These are helpful in the regulation of cell polarization and motility of Dictyostelium towards chemoattractant. Because of this reason solo mutants (which is unable to bind downstream targets) cannot effectively polarize and shows poor chemotaxis. So these cells will not be polarized in the presence of cAMP gradient and mover very slowly and inefficiently towards the cAMP source.

(ii)In case of Clumpy mutants, G unit is unable to interact with G. But the G complex can couple with cAR1 and can produce the response. The G subunit is constitutively active in this case and it is essential for chemotaxis towards all chemoattractant. The cell was able to localize towards the chemoattractant.

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