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Updated: Feb 28, 2026
Examining the Conformational Dynamics of Membrane Proteins in situ with Site-directed Fluorescence Labeling
Published on: May 29, 2011
Conformational transition of κ-casein in micellar environment: Insight from the tryptophan fluorescence
Smruti Mishra1, Geetanjali Meher1, Hirak Chakraborty1
1School of Chemistry, Sambalpur University, Jyoti Vihar, Burla, Odisha 768 019, India.
Insights
Intrinsically disordered proteins like kappa-casein change shape when interacting with surfactants. Positively charged surfactants, such as cetyltrimethyl ammonium bromide (CTAB), induce partial folding at low concentrations and unfolding at high concentrations.
Area of Science:
- Biochemistry
- Protein Science
- Biophysical Chemistry
Background:
- Intrinsically disordered proteins (IDPs) lack stable tertiary structures, making them sensitive to environmental changes.
- Conformational changes in IDPs can lead to aggregation and cellular dysfunction.
- Milk proteins like kappa-casein are IDPs crucial for biological functions.
Purpose of the Study:
- To investigate the interaction between kappa-casein and the cationic surfactant cetyltrimethyl ammonium bromide (CTAB).
- To elucidate the conformational changes of kappa-casein induced by varying CTAB concentrations.
- To understand the role of electrostatic and surfactant properties in protein-surfactant interactions.
Main Methods:
- Steady-state fluorescence spectroscopy
- Time-resolved fluorescence spectroscopy
- Circular dichroism spectroscopy
Main Results:
- Kappa-casein exhibits at least two distinct conformational transitions upon interaction with CTAB.
- At low CTAB concentrations, kappa-casein adopts a partially folded state, likely due to electrostatic interactions.
- At high CTAB concentrations, kappa-casein becomes unstructured, indicating the dominance of surfactant properties.
Conclusions:
- The study reveals the complex conformational behavior of kappa-casein in response to a cationic surfactant.
- Electrostatic interactions play a key role in the initial conformational changes of kappa-casein with CTAB.
- The findings highlight the sensitivity of IDPs to microenvironmental modifications, particularly from surfactants.
Abstract:
Intrinsically disordered proteins (IDPs) are under intense analysis due to their structural flexibility and importance in biological functions. Minuscule modulation in the microenvironment induces significant conformational changes in IDPs, and these non-native conformations of the IDPs often induce aggregation and cause cell death. Changes in the membrane composition often change the microenvironment, which promote conformational change and aggregation of IDPs. κ-Casein, an important milk protein, belongs to the class of IDPs containing net negative charges. In this present work, we have studied the interaction of κ-casein with cetyltrimethyl ammonium bromide (CTAB), a positively charged surfactant, utilizing various steady state fluorescence, time-resolved fluorescence and circular dichroism spectroscopy. Our results clearly indicate that κ-casein undergoes at least two conformational transitions in presence of various concentrations of CTAB. The intrinsically disordered κ-casein assumes a partially folded conformation at lower concentration of CTAB, which adopts an unstructured conformation at higher concentration of CTAB. The partially folded conformation of κ-casein at a lower CTAB concentration might be induced by the favorable electrostatic interaction between the positively charged surfactant headgroup and net negative charges of the protein, whereas surfactant nature of CTAB is being pronounced at higher concentration of CTAB.
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