Gelsolin is activated by Ca2+ to sever actin filaments. Ca2+ regulation is conferred on the N-terminal half by the C-terminal half. This paper seeks to understand how Ca2+ regulates gelsolin by testing the 'tail helix latch hypothesis,' which is based on the structural data showing that gelsolin has a C-terminal tail helix that contacts the N-terminal half in the absence of Ca2+. Ca2+ activation of gelsolin at 37 °C occurs in three steps, with apparent K(d) for Ca2+ of 0.1, 0.3, and 6.4 x 10-6 M. Tail helix truncation decreases the apparent Ca2+ requirement for severing to 10-7 M and eliminates the conformational change observed at 10-6 M Ca2+. The large decrease in Ca2+ requirement for severing is not due to a change in Ca2+ binding nor to Ca2+-independent activation of the C-terminal half per se. Thus, the tail helix latch is primarily responsible for transmitting micromolar Ca2+ information from the gelsolin C-terminal half to the N-terminal half. Occupation of submicromolar Ca2+-binding sites primes gelsolin for severing, but gelsolin cannot sever because the tail latch is still engaged. Unlatching the tail helix by 10-6 M Ca2+ releases the final constraint to initiate the severing cascade.
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