Inhibitory effects of poly(L-aspartic acid) on the assembly of brain microtubules and the interaction of microtubule-associated protein 2 with F-actin in vitro.

The effects of poly(L-aspartic acid) (PLAA) on microtubule assembly and microtubule-associated protein (MAP) 2-actin interaction were examined in vitro. PLAA inhibited assembly of rat brain microtubules and induced rapid disassembly of already formed microtubules. Inhibition was stronger by PLAA with a high molecular weight than that by low molecular weight. The ratios of 47 kDa PLAA to microtubule proteins causing 50% inhibition of the assembly and disassembly were 0.015 and 0.04 (w/w), respectively. Both MAP 1 and MAP 2 were bound to a PLAA-Sepharose 4B affinity column, while tubulin was not retained by the column. PLAA caused selective dissociation of MAP 1 and MAP 2 from microtubules polymerized by taxol. It is therefore concluded that PLAA interacts specifically with MAPs. PLAA also inhibited the MAP 2 induced cross-linking of actin filaments, showing an almost complete inhibition at a PLAA to MAP 2 ratio of 1:5,000 (w/w). Binding experiments of PLAA with digested MAP 2 by chymotrypsin using affinity chromatography and sedimentation experiments showed that PLAA was preferentially bound to a 35 kDa fragment which includes the microtubule- and actin-binding domain of the MAP 2 molecule. These results suggest that PLAA suppressed the functions of MAP 2 through a domain which is located in the 35 kDa fragment.