Summary To maintain mechanical homeostasis, cells must recognize and respond to changes in cytoskeletal integrity. By imaging live cells expressing fluorescently tagged cytoskeletal proteins, we observed that actin stress fibers undergo local, acute, force-induced elongation and thinning events that compromise their stress transmission function, followed by stress fiber repair that restores this capability. The LIM protein, zyxin, rapidly accumulates at sites of strain-induced stress fiber damage and is essential for stress fiber repair and generation of traction force. Zyxin promotes recruitment of the actin regulatory proteins, α-actinin and VASP, to compromised stress fiber zones. α-Actinin plays a critical role in restoration of actin integrity at sites of local stress fiber damage, while both α-actinin and VASP independently contribute to limiting stress fiber elongation at strain sites, thus promoting stabilization of the stress fiber. Our findings demonstrate a mechanism for rapid repair and maintenance of the structural integrity of the actin cytoskeleton.

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The yeast Zap1 transcription factor controls the expression of genes involved in zinc accumulation and storage. Zap1 is active in zinc-limited cells and repressed in replete cells. Zap1 has two activation domains, AD1 and AD2, which are both regulated by zinc. AD2 function was mapped to a region containing two Cys2His2 zinc fingers, ZF1 and ZF2, that are not involved in DNA binding. More detailed mapping placed AD2 almost precisely within the endpoints of ZF2, suggesting a role for these fingers in regulating activation domain function. Consistent with this hypothesis, ZF1 and ZF2 bound zinc in vitro but less stably than did zinc fingers involved in DNA binding. Furthermore, mutations predicted to disrupt zinc binding to ZF1 and/or ZF2 rendered AD2 constitutively active. Our results also indicate that the repressed form of AD2 requires an intramolecular interaction between ZF1 and ZF2. These studies suggest that these zinc fingers play an unprecedented role as zinc sensors to control activation domain function.