Figure 10.20 Testing the Signal
For proper organelle localization, intracellular signaling is required, directing polypeptides to the correct destination. Often such signaling involves a localization signal or tag, which is a specific amino acid sequence that interacts with a receptor protein on the surface of the organelle that is the correct “destination.” Dingwall and colleagues investigated the nuclear localization signal used by the nuclear protein nucleoplasmin by experimentally manipulating both this nuclear protein, as well as the cytoplasmic protein pyruvate kinase. The researchers first determined that a specific “tail” fragment of amino acids, not the main core part of nucleoplasmin, was both necessary and sufficient for localization from the cytoplasm to the nucleus. This was experimentally determined in monkey cells by injecting nucleoplasmin with and without the “tail” and observing localization using immunofluorescence microscopy. The nuclear localization function of this “tail” was then tested by injecting into cells the normal cytoplasmic protein pyruvate kinase, as well as pyruvate kinase linked to the nucleoplasmin “tail.” As predicted, the normal pyruvate kinase remained in the cytoplasm, whereas the pyruvate kinase linked to the nucleoplasmin “tail” entered the nucleus. Intracellular trafficking is important for many polypeptides, including those destined for mitochondria or chloroplasts. To study protein import into chloroplasts, Schnell and colleagues used immunofluorescence microscopy and synthetic peptides, determining that the signal polypeptides bind to specific receptor proteins on the chloroplast surface. Recently, defects in protein localization have been implicated in several human diseases. For example, inclusion-cell disease is due to a lack of the necessary localization signal for transport of certain proteins from the Golgi apparatus into the lysosomes, and Zellweger syndrome results from a mutated receptor protein, inhibiting proteins from entering the peroxisome.
Original Paper
Dingwall, C., J. Robbins, S. M. Dilworth, B. Roberts, and W. D. Richardson. 1988. The Nucleoplasmin Nuclear Location Sequence is Larger and More Complex than That of SV-40 Large T Antigen. The Journal of Cell Biology 107: 841–849.
Links
Schnell, D. J., G. Blobel, and D. Pain. 1991. Signal peptide analogs derived from two chloroplast precursors interact with the signal recognition system of the chloroplast envelope. Journal of Biological Chemistry 266: 3335–3342.
Wikipedia: Nuclear localization signal
University College London: K. Singh Dulai: Signal peptides
Kimball’s Biology Pages: Protein Kinesis: Getting Proteins to Their Destination
Wikipedia: Inclusion-cell disease
NCBI: Genes and Disease: Zellweger syndrome
University of Utah: Genetic Science Learning Center: Amazing Cells