Adding to the growing fundamental understanding of the machinery of muscle cells, a group of biophysicists describe -- in minute detail -- how actin filaments are stabilized at one of their ends to form a basic muscle structure called the sarcomere. With the help of many other proteins, actin molecules polymerize to form filaments that give rise to structures of many different shapes. The actin filaments have a polarity, with a plus and minus end, reflecting their natural tendency to gain or lose subunits when not stabilized.

Source: Atomic structure of key muscle component revealed