The QDC group shows that QDs can be used to label proteins in fixed and living cells. Using two colors of IgG- and streptavidin-attached QDs, they simultaneously identified both nuclear and plasma membrane proteins with only a single filter set in fixed cells. QD emissions were four to nine times brighter than Alexa, one of the brightest organic dyes.
Simon's group extended the usage to long-term labeling of living cells. QDs covered with a stable chemical coat were taken up by endocytosis or were linked to antibodies to label specific membrane proteins. As the label did not affect cell growth or development, the stability of QD fluorescence allowed the authors to follow developing amoebae over several hours and mammalian cells for over 12 d. Simon looks forward to using QD technology to examine protein dynamics during vesicle fusion to the plasma membrane, a process not well suited to the rapidly photobleached GFP.
In the future, nonendocytic organelles may be specifically labeled by conjugating QDs to a signal peptide that will target them to the desired organelle. Labeling of specific nonmembrane-associated proteins inside a living cell may be done by conjugation of the QDs to protein-specific ligands or antibodies. However, for now, those labels will have to be delivered into the cell using the same invasive techniques as for conventional membrane-impermeant dyes. ▪