© 2005 The Japanese Biochemical Society.
Regular Paper |
Role of the Basic C-Terminal Half of Caldesmon in Its Regulation of F-Actin: Comparison between Caldesmon and Calponin
Department of Molecular Biology and Biochemistry, Nelson Labs/Busch Campus, Rutgers, The State University of New Jersey, USA
* To whom correspondence should be addressed at the present address: Division of Molecular Biology, School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602. Phone: +81 52 789 3542, Fax: +81 52 789 3001, E-mail: j46037a{at}nucc.cc.nagoya-u.ac.jp
We previously reported that caldesmon (CaD), together with tropomyosin (TM), effectively protects actin filaments from gelsolin, an actin-severing protein. To elucidate the structure/function relationship of CaD, we dissected the functional domain of CaD required for the protection. The basic C-terminal half of rat nonmuscle CaD (D3) inhibits gelsolin activity to the same degree as intact CaD, although a smaller C-terminal region of D3 does not. This smaller C-terminal region contains the minimum regulatory domain responsible for the inhibition of actomyosin ATPase, and for the binding to actin, calmodulin and TM. These results suggest that the domain responsible for the inhibition of gelsolin activity lies outside the minimum regulatory domain, and that the positive charge possessed by the C-terminal half of CaD is important for its interaction with actin. Moreover, while the D3 fragment promotes the aggregation of F-actin into bundles as reported previously, this bundle formation is inhibited by the acidic N-terminal half of CaD, as well as by poly-L-glutamate. It seems likely that the acidic N-terminal half of CaD neutralizes the superfluous basic feature of the C-terminal half. A comparison between D3 and calponin, another actin-binding protein that is also basic and has similar actin-regulatory activities, is also discussed.