Microfilaments are organized into higher-order structures, forming bundles or networks. Because all the actin monomers are oriented in the same direction, actin filaments have a distinct polarity and their ends, called the plus and minus ends. This polarity of actin filaments is important both in their assembly and in establishing a unique direction of myosin movement relative to actin.
Actin filaments are then able to grow by the reversible addition of monomers to both ends.
The plus end elongates about 10 times faster that the minus end. Organization of Actin Filaments Actin bundles are actin filaments crosslinked into parallel arrays closely packed. The crosslinking proteins are called actin bundling proteins . Fimbrin is an example wihich occurs in actin bundles forming microvilli. Alfa-actinin occurs in the contractile bundles.