1. Proteins are imported into organelles through pores (nucleus), protein translocators, or transport vesicles. (look)

• Proteins enter through nuclear pores by attaching to fibers around the pore, aided by nuclear import receptor proteins found in the cytosol. This process is active transport powered by hydrolysis of GTP. (look) , (look) , (look)
• Proteins unfold to enter mitochondria and chloroplasts. Remember that such unfolding is aided by molecular chaperones. (look)
• In a similar process, soluble proteins destined for the endoplasmic reticulum (ER) interior (lumen) are synthesized by ribosomes located on the ER membrane. Molecular chaperones help the protein fold up as it is being extruded into the lumen of the ER. The first part of the protein sequence has a single peptide which is specific for binding to the ER channel. A signal peptidase later removes the signal peptide. (look), (look)
• Transmembrane proteins destined to be incorporated into membranes are synthesized like proteins destined for the lumen of the ER except that the protein incorporates a hydrophobic sequence or sequences which anchor the protein in the membrane. (look), (look)
• Some proteins are moved between the cytosol and the ER, vesicles, and the Golgi apparatus by clathrin coated vesicles. (look), (look), (look)
• Vesicles find their targets by transmembrane proteins called SNARES. (look), (look)
• Some proteins are covalently modified, often by addition of polysaccharide changes in the ER and in the Golgi. (look), (look)
• In summer, 2006, a paper was published in Nature Methods that describes the use of fluorescent probe molecules to distinguish between two hypotheses of how the Golgi apparatus processes proteins.: "Microscopy matures, and so does the Golgi" (This short news item is required reading.)
• Exocytosis releases proteins. (look)

2. Lysosomes

• Lysosomes are vesicles bound by a single membrane. They appear in a tremendous range of sizes, ranging in diameter from 50 nm, the size of the smallest mycoplasmal cell to as much as 0.5 micrometers. Lysosomes are manufactured by the rough endoplasmic reticulum and then fuse with injested material or cell components which are to be broken down and recycled. (look). An article in the Journal of Cell Biology describes the role that chance and accident played in understanding lysosomal function.
• The lysosome contains enzymes to break down just about everything. These enzymes operate under acid conditions resulting from H⁺ pumping from the cytosol. The necessity of having acid conditions protects the cell interior in case of a leak. (look)
• Failure of the lysosomal system can lead to serious consequences:
• Silicosis and asbestosis - Silica, such as found in mines, and asbestos are minerals which consist of sharp needles. These are endocytosed into cells lining the lungs and the vesicles fused with lysosomes. But, the enzymes in the lysosome can't digest the mineral materials. Worse yet, the needle like shapes of the injested particles puncture the fused vesicles, releasing enzymes into the cell interior.
• With lysosomal storage diseases, one or more of the enzymes found in the lysosome is missing. So, old mitochondria and other organelles or their components just build up, filling the cell and causing cell death. Tay-Sachs disease is due to a deficiency in an enzyme which breaks down a particular kind of fat called hexosaminidase A. These lipids build up in nerve cells and cause cell death, leading to brain damage, usually with death by 3 years of age. Another lysosomal storage disease is Gaucher's which is due to a deficiency in glucocerebrosidase. Gaucher's disease leads to liver and spleen enlargement and erosion of bones. There can be brain damage.
• Rheumatoid arthritis is an autoimmune disease, where the immune system attacks the body's own cells. One of the consequences of this disease is that some cells release their lysosomal contents into joints, etc, destroying cartilage.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.