Christmas in July

First there was green fluorescent protein (GFP) from jellyfish, then came red fluorescent protein (DsRed) from coral. Now researchers from the University of California at Irvine show that DsRed may become equally indispensable for tracking cells, organelles, and fusion proteins by turning them… green?

Ian Parker says he and his colleagues made the discovery quite by accident while exploring the use of DsRed for multiphoton confocal imaging. Brief exposure to a femtosecond-pulsed laser beam turned DsRed's fluorescence from red to green when viewed with a conventional one-photon microscope. The excitation, they explain, selectively bleaches the mature, red-emitting form of DsRed, which enhances emission from the immature green form.

The color change persists for hours or even days, and appears to do the cells no damage. Parker likens the technique to using a highlighter pen to mark important bits of text. The color change, he says, provides a powerful tool for picking out individual cells, fusion proteins, or even small regions of cells, by bleaching them green and leaving red everywhere else. Thus, the technique is suitable for studying cell lineages, organelle dynamics, and protein trafficking, as well as for selective retrieval of cells from a population.

Another method for labeling part of a cell has been described by Shuichi Takayama and colleagues (Harvard University, Cambridge, MA). They developed a chip that uses multiple laminar streams in a microfluidic channel; it can deliver membrane-permeable molecules to selected bits of a cell without causing major disruptions.

The researchers are calling the method “partial treatment of cells using laminar flows,” or PARTCELL, and have used it to study mitochondrial movement and changes in cytoskeletal structure. One example: they streamed different solutions containing fluorescent tags over opposite poles of a live bovine capillary endothelial cell. This enabled them to selectively label mitochondrial subpopulations in different parts of the cell and watch the two kinds of mitochondria moving around and mixing together. ▪

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