Actin filaments with a-actinin vs fimbrin

Actin filaments can be bundled into tight or loose bundles with either the same or mixed polarity. For example a-actinin form dimers which bundle either parallel or antiparallel filaments in contractile bundles. In muscle the filaments are always arranged in a parallel fashion. The distance between the bundles allows myosin to get to the actin filament. By contrast fimbrin only bundles filaments of the same polarity and packs them more tightly. This is observed in structures such as microvilli.