Monitoring the aggregation of single casein micelles using fluorescence microscopy

Investigations on the Effect of Fatty Acid Additives on Casein Micelles: Role of Ethylenic Unsaturation on the Interaction and Structural Diversity

Casein, one of the major constituent of milk protein, is considered to be a good candidate for oral drug delivery system. Also, milk transports various essential fatty acid to blood through dietary supplements. In this study, we have explored the alteration in the structural characteristic in terms of the modulations in the microenvironment of the protein in the presence of different types of fatty acids. Herein, we have observed that the unsaturation of fatty acids mostly affects the structure of casein micelles (CMs) by impinging upon the hydrophobic force of interaction following a decrease in the electrostatic interaction of various amino acid unit. Alteration of such forces is responsible for the increase in the aggregate size, modification in the protein secondary structure, and different morphology of CMs. Fluorescence behavior of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran indicates that the rigidity of the microenvironment is the main characteristic of the fatty acid binding, and the binding constant increases with the fatty acid chain length for saturated fatty acid or with the introduction of unsaturation onto it. Fluorescence lifetime imaging microscopy study indicates that the microstructure of CMs becomes more compact in the presence of unsaturated fatty acids, and this is also responsible for the increase in the diffusion time of the probe. Moreover, decrease in the fluorescence of extrinsic probe 8-anilinonaphthalene-1-sulfonate with the addition of unsaturated fatty acid reveals that these fatty acids alter the electrostatic interaction between casein units, more specifically in case of the surface-bound κ-casein. Therefore, this study provides a very useful information on the binding of fatty acids and helps to evaluate other fatty acid, as well as different small molecules binding in the applicative medicinal purpose.

Nanoscale high-content analysis using compositional heterogeneities of single proteoliposomes

Proteoliposome reconstitution is a standard method to stabilize purified transmembrane proteins in membranes for structural and functional assays. Here we quantified intrareconstitution heterogeneities in single proteoliposomes using fluorescence microscopy. Our results suggest that compositional heterogeneities can severely skew ensemble-average proteoliposome measurements but also enable ultraminiaturized high-content screens. We took advantage of this screening capability to map the oligomerization energy of the β2-adrenergic receptor using ∼10(9)-fold less protein than conventional assays.

AFM imaging of milk casein micelles: evidence for structural rearrangement upon acidification

Milk casein micelles are natural association colloids that we all encounter in everyday life, yet we still lack an accurate description of their internal structure and the interactions that stabilize it. In this letter, we provide for the first time detailed images of intact casein micelles as obtained through atomic force microscopy under liquid conditions close to physiological. The micelles appear as heterogeneous raspberry-like particles, which is consistent with a hierarchical/spongelike structure made of connected 10-40 nm dense casein regions. Upon in situ acidification to pH 5, the micelles decrease in size and lose their surface heterogeneities, indicating that this structure is highly sensitive to variations in mineral content and caseins net charge.