Proposed model for conversion of unipolar to bidirectional actin filaments during the maturation of SFs. Unipolar, α-actinin cross-linked actin filament bundles oriented with their barbed ends facing the focal adhesion are first severed and capped (step 1). This severing protein then either recruits another protein or protein complex that nucleates actin filament polymerization in the opposite orientation, or alternatively, a single protein possessing severing/capping/nucleation activity may fulfill this role (step 2). The final stage involves incorporation of myosin filaments into the maturing SF with its characteristic periodic distribution (step 3).