TRANSIENT ELECTRIC BIREFRINGENCE STUDY OF INTERMEDIATE FILAMENT FORMATION FROM VIMENTIN AND GLIAL FIBRILLARY ACIDIC PROTEIN

Mg2+-induced polymerization of type III intermediate filament proteins vimentin and glial fibrillary acidic protein was studied by transient electric birefringence. In the absence of MgCl2 we found a net perman ent dipole moment, similar to 45-nm-long dimers for vimentin, similar to 65-nm-long tetramers, hexamers, and possibly octamers for both prot eins, and 100-nm aggregates for glial fibrillary acidic protein. Contr olled oligomerization occurred after the addition of MgCl2. Although t he solutions contained (small) aggregates of different sizes, more or less discrete steps in polymer formation were observed, and it was pos sible to discriminate between an increase in width and length. At the first stage of polymerization (in 0.3 mM MgCl2 for vimentin and 0.2 mM MgCl2 for glial fibrillary acidic protein), the permanent dipole mome nt disappeared without a change in length of the particles. At higher MgCl2 concentrations, structures of approximately 100 nm were formed, which strongly tended to laterally assemble into full-width intermedia te filament structures consisting of about 32 monomers. This contrasts with previous models where first full-width (similar to 10-nm) aggreg ates are formed, which then increase in length. Subsequently, two disc rete elongation steps of 35 nm are observed that increase the length t o 135 and 170 nm, respectively. Possible structural models are suggest ed for the polymerization.