Hydrodynamic and Electrical Characterization of T-vimentin Dimers and Tetramers by Transient Electric Birefringence Measurements (∗)

The structure and charge distribution of T-vimentin, which differs from the intact intermediate filament protein vimentin through the absence of the first 70 amino acids, has been studied by transient electric birefringence measurements. It is found that in 0.7 mM phosphate, pH 7.5 buffer, exclusively single dimers (with a hydrodynamic length of 40 to 43 nm) are present, which are considerably bent and/or flexible and which have a relatively large permanent dipole moment. This indicates a parallel alignment of two protein chains. In 0.2 mM phosphate, 0.5 mM MgCl2, pH 7.5, predominantly tetrameric T-vimentin is found with a rigid structure, no permanent dipole moment, and a length of 63 to 68 nm. Tetramer formation is likely to be induced by binding of Mg2+ to the protein. The observed length is in agreement with that of intact vimentin tetramers in which the 1B regions of the rod domains of the dimers overlap (A11 configuration). A minor part of the tetramers may be in a flexible or bent A22 form. The loss of the permanent dipole moment when tetramers are formed is, apart from charge compensation, presumably due to the antiparallel alignment of the constituting dimers in which their dipoles cancel.