Interpretation of Resonance Energy Transfer Between Protein Subunits. Figure 1.3

Question

Interpretation of Resonance Energy Transfer Between Protein Subunits. Figure 1.39 shows emission spectra of fluorescein (FI) and rhodamine (Rh) when covalently attached to the catalytic (C) or regulator (R) subunit of a cAMPdependent protein kinase (PK) When the subunits-are associated, RET occurs from FI to Rh The associated form is C2R2. Figure 1.40 shows emission spectra of both subunits without any additives, in the presence of cAMP, and in the presence of protein kinase inhibitor (PKI). Suggest an interpretation of these spectra. Your interpretation should be consistent with the data, but it may not be the only possible interpretation.

Explanation / Answer

Fluorescence in presence of PKI reveals that there is binding of PKI to the regulatory domain of cAMP dependent protein kinase. It is because the fluorescence is quenched at that region. The binding results the inhibition of catalytic property of cAMP dependent protein kinase by binding at regulatory domain.

The cAMP presence should make low fluorescence at regulatory domain and that is already happening in the graph. It is because cAMP is a regulatory protein. So binding to it results into quenching of fluorescence.

The increase in catalytic domain fluorescence is due to binding of free FI to catalytic domain of cAMP dependent protein kinase which get opened after binding of additives. This is conformational change.

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