Transport to Peroxisomes

Most organelles that are bound by a single membrane have their proteins made at the rough endoplasmic reticulum and transported to them in vesicles (Fig. 10.1). Peroxisomes are an exception: their proteins are synthesized on free-floating polyribosomes and then transported to their final destination. Peroxisomal targeting sequences on the protein bind to peroxisome import receptors in the cytosol. The complex of cargo and receptor docks onto the peroxisomal membrane and then crosses the membrane to enter the peroxi-some. Here, the protein cargo is released and the import receptor is shuttled back into the cytosol.

Medical Blocking Calcineurin—How Immunosuppressants Work Relevance The drug cyclosporin A is invaluable in modern medicine because it suppresses the 10.1 immune response that would otherwise cause the rejection of transplanted organs. It does this by blocking a critical stage in the activation of T lymphocytes, one of the cell types in the immune system. T lymphocytes signal to other cells of the immune system by synthesizing and releasing the protein interleukin 2. Transcription of the interleukin 2 gene is activated by a transcription factor called NFAT.

plasmalemma foreign protein -1 Receptor cytoplasm z inactive calcineurin foreign protein -1 Receptor

calmodulin cyclosporin nuclear localization sequence

dephosphorylated NFAT

phosphorylated NFAT

nucleus

NFAT binding site in enhancer region of interleukin 2 gene

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