HYDRODYNAMIC AND ELECTRICAL CHARACTERIZATION OF T-VIMENTIN DIMERS ANDTETRAMERS BY TRANSIENT ELECTRIC BIREFRINGENCE MEASUREMENTS

The structure and charge distribution of T-vimentin, which differs fro m the intact intermediate filament protein vimentin through the absenc e 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 an d which have a relatively large permanent dipole moment. This indicate s 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 v imentin tetramers in which the 1B regions of the rod domains of the di mers overlap (A(11) configuration). A minor part of the tetramers may be in a flexible or bent A(22) form. The loss of the permanent dipole moment when tetramers are formed is, apart from charge compensation, p resumably due to the antiparallel alignment of the constituting dimers in which their dipoles cancel.