INFO-GRAPHIC,,Johnstone Quinan; A NEW MODEL FOR ALZHEIMER'S DISEASE One of the most baffling of human afflictions may be explained by a new theory on the behavior of certain long-mysterious protein molecules that reside in the membranes enclosing brain cells. Nobody knows why cells make the molecules but it is widely accepted that they are involved in Alzheimer's disease - either as a cause or an effect. 1. What normally happens is that cells make a protein called APP and install it in their membranes. Most of the protein hangs outside the cell but is anchored by a part of the molecule (cylindrical spiral) whose chemistry holds it in the membrane. 2. Normally an enzyme clips the APP close to the anchor, leaving a molecule called amyloid. The freed segment floats away and is thought to play no further role. 3. In Alzheimer's disease an enzyme clips the APP farther out, leaving more of the zig-zag as part of the amyloid. 4. The new model holds that several of the amyloids form a ring and turn their zig-zag parts into the space within the ring. Experiments show that when this happens in artificial membranes, the ring acts as a "calcium channel" and pumps calcium ions into the cell. Excess calcium can kill the cell.