Direct Vpr-Vpr interaction in HeLa cells visualized by FLIM. Cells were transfected with the DNA construct encoding eGFP or eGFP-Vpr alone or in combination with mCherry-Vpr. In the FLIM images, the lifetimes are represented using an arbitrary color scale ranging from blue to red for short and long lifetimes in nanoseconds (right bottom), respectively. The Vpr-eGFP or eGFP-Vpr with short lifetime fluorescence symbolized by the blue color were mainly localized at the nuclear envelope and also in other cell compartments when co transfected with mCherry tagged Vpr. Panels A1 to A3 show the lifetime images of cells expressing eGFP or eGFP-tagged Vpr alone. Panels B1 and B2 represent cells coexpressing eGFP-tagged Vpr and mCherry; Panels B3 and C1-C3 show the lifetime images of cells coexpressing eGFP-tagged Vpr and mCherry-tagged Vpr. Note the accumulation of Vpr fusion proteins at or near the nuclear envelope.