Ca2+ regulation of gelsolin by its C-terminal tail

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 t o understand how Ca2+ regulates gelsolin by testing the "tail helix latch h ypothesis," 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 o f Ca2+. Ca2+ activation of gelsolin at 37 degrees 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 trun cation 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+ bi nding nor to Ca2+-independent activation of the C-terminal half per se. Thu s, the tail helix latch is primarily responsible for transmitting micromola r Ca2+ information from the gelsolin C-terminal half to the N-terminal half . Occupation of submicromolar Ca2+-binding sites primes gelsolin for severi ng, but gelsolin cannot sever because the fail latch is still engaged. Unla tching the tail helix by 10(-6) M Ca2+ releases the final constraint to ini tiate the severing cascade.