Calcium regulation of gelsolin and adseverin: A natural test of the helix latch hypothesis

The gelsolin family of actin filament binding proteins have highly homologo us structures. Gelsolin and adseverin, also known as scinderin, are the mos t similar members of this family, with adseverin lacking a C-terminal helix found in gelsolin. This helix has been postulated to serve as a calcium-se nsitive latch, keeping,oelsolin inactive. To test this hypothesis, we have analyzed the kinetics of severing by gelsolin, adseverin, and a gelsolin tr uncate which lacks the C-terminal latch. We find that the relationship betw een severing rate and calcium ion concentration differs between,aelsolin an d adseverin, and suggest that calcium controls one rate-limiting step in th e activation of adseverin and two in the activation of adseverin and two in the activation of gelsolin. In contrast, both proteins are activated equal ly by protons, and have identical severing kinetics at pHs below 6.3. The t emperature sensitivity of severing by adseverin and gelsolin is remarkably different, with gelsolin increasing its severing rate 8-fold per 10 degrees C increase in temperature and adseverin increasing its rate only 2-fold pe r 10 degrees C increase in temperature. Analysis of the gelsolin construct lacking the C-terminal helix demonstrates that this helix is responsible fo r the regulatory differences between gelsolin and adseverin. These results support the C-terrninal latch hypothesis for the calcium ion activation of gelsolin.