Quantifying protein-protein interactions of peripheral membrane proteins by fluorescence brightness analysis.

Fluorescently labeled proteins that are found both in the cytoplasm and at the plasma membrane, such as peripheral membrane proteins, create stratified fluorescent layers that present a challenging environment for brightness studies with fluorescence fluctuation spectroscopy. The geometry of each layer along with fluorescence and brightness contributions from adjacent layers generates a convoluted raw brightness that conceals the underlying brightness of each individual layer. Because the brightness at a layer establishes the oligomeric state of the fluorescently labeled protein at said layer, we developed a method that connects the experimental raw brightness with the physical brightness at each layered compartment. The technique determines the oligomerization in each compartment from an axial intensity scan through the sample, followed by a fluorescence fluctuation spectroscopy measurement at each layer. We experimentally verify the technique with H-Ras-EGFP as a model system and determine its oligomeric state at both the plasma membrane and in the cytoplasm. Furthermore, we study the oligomerization of the Gag matrix domain of Human T-lymphotropic virus Type 1. The matrix domain targets the Gag polyprotein to the plasma membrane where, subsequently, viral assembly occurs. We determine the oligomerization of matrix in the cytoplasm and observe the onset of protein-protein interactions at the membrane. These observations shed light on the early assembly steps of the retrovirus.