Protein complex helps clotting.

• Position: Circulatory System - Arp2/3 medical research - Brief Article

Scientists at Brown University, Providence, R.I., have discovered a previously unknown, but critical, blood-clotting role for Arp2/3, a complex of seven proteins found in animal and plant cells. The Arp2/3 complex is a cellular machine that drives a human blood platelet to change shape into a larger, more-flattened form and begin the process of clotting. The link between what happens at the surface of a platelet and the mechanism of shape change within it has mystified scientists for decades.

"A major question for scientists has been how to control platelet shape change," indicates Elaine Bearer, associate professor of pathology and laboratory medicine. "Understanding these molecular events could lead to better treatments for abnormal clotting." Roughly 80% of strokes are caused by atypical clots that block blood flow.

Bearer and colleagues found that Arp2/3 complex is required for platelets to form the shape-changing filaments that begin the blood-clotting process. The process of filament formation is called actin polymerization. Actin filaments are fine threads composed of multiple subunits--polymers that line up like a string of pop beads--which give structure to the cell, as well as drive shape changes, cell movements, and other cellular processes.

Since the first cell was observed under a microscope more than 200 years ago, scientists have sought to explain cellular shape change. Cell shape is used today for the pathologic diagnosis of tumors such as breast cancer. For 50 years, scientists have known that actin was required for shape change. Until now, they were unable to explain what drove actin to polymerize and form filaments...