Microtubule-associated proteins, MAP 1A and MAP 1B, interact with F-actin in vitro.

Microtubule-associated protein (MAP) 1 consisting of MAP 1A and 1B was purified from rat brain by the poly-L-aspartic acid (PLAA) method. We found that MAP 1 bound to F-actin in vitro up to a molar ratio of MAP 1 to actin monomers of 1:10. The apparent binding constant was about 2.7 x 10(7) M-1. In contrast to the binding of MAP 2 or tau to F-actin, the binding of MAP 1 to F-actin did not affect the low-shear viscosity of actin filaments. Binding experiments performed using fragments of MAP 1, obtained by chymotrypsin digestion, indicated that MAP 1 included binding domains to F-actin that were different from those in microtubules and also two light chains (31 and 29 kDa) that were cosedimented with F-actin as well as with microtubules.