Calmodulin-regulated binding of the 90-kDa heat shock protein to actin filaments

We have found that the 90-kDa heat shock protein (HSP90) prepared from a mouse lymphoma exists in homodimeric form under physiological conditions and has the ability to bind to F-actin (Koyasu, S., Nishida, E., Kadowaki, T., Matsuzaki, F., Iida, K., Harada, F., Kasuga, M., Sakai, H., and Yahara, I. (1986) Proc. Natl. Acad. Sci. U.S.A., in press). Here we show that calmodulin regulates the binding of HSP90 to F-actin in a Ca2+-dependent manner. The binding of HSP90 to F-actin occurred optimally under physiological solution conditions, i.e. in 2 mM MgCl2 + 100 mM KCl. The binding was saturable in a molar ratio of about 1 HSP90 (dimer) to 10 actins. HSP90 was dissociated from F-actin by the binding of tropomyosin to F-actin. Calmodulin was found to inhibit the binding of HSP90 to F-actin in a Ca2+-dependent manner. Moreover, the equilibrium gel filtration demonstrated that calmodulin binds to HSP90 in the presence of Ca2+, but not in the absence of Ca2+. These data indicate that HSP90 complexed with Ca2+-calmodulin is unable to bind to F-actin. Ca2+-dependent interaction of HSP90 with calmodulin as well as calmodulin-regulated binding of HSP90 to F-actin revealed here may provide new insight into the function of HSP90 and the regulation of actin structure in cells.