Fig. 6. Ultrastructural analysis of F-actin/Xin-repeat complexes. Bacterially expressed XR1-11 was purified and its capability to cross link skeletal-muscle F-actin was investigated using low-speed co-sedimentation assays. (A,B) Lanes 1 show the supernatant or pellet fractions of mixtures of XR1-11 and F-actin upon centrifugation. The numbers on top of each lane indicate the amount of recombinant protein that was added to 5 µM actin. The positions of actin and XR1-11 are marked with an asterisk and an arrow, respectively. To investigate the organization of the cross-linked actin filaments, F-actin alone (C,D) or mixtures of actin and XR1-11 (E,F) were applied to carbon-coated Formvar grids and analysed by electron microscopy after negative stain. The Xin-repeat construct does not bundle actin filaments but, instead, seems to cross-link F-actin into a meshwork (E,F). Scale bars, 500 nm (CE), 100 nm (D,F).