An atomic model of crystalline actin tubes: combining electron microscopy with X-ray crystallography.

The packing of the G-actin monomers within crystalline actin tubes was investigated at atomic detail. To achieve this, we have chosen an integrated structural approach which combines intermediate resolution electron microscopy based 3-D reconstruction and surface metal shadowing of crystalline actin tubes with atomic resolution X-ray data of the G-actin monomer. Distinct from the parallel, half-staggered packing of the actin subunits within F-actin filaments, the arrangement of actin monomers within the crystalline tubes involves antiparallel packing into dimers with p2 symmetry. Within the crystalline tubes, the actin monomers are oriented so that the filament axis runs parallel with the sheet plane and the intersubunit contacts in this direction are similar to those existing along the two long-pitch helical strands of the F-actin filament. The other intersubunit contacts within the crystalline tubes are not found in the actin filament. The ability of actin to form a variety of polymorphic oligomers is still not fully understood, and the functional implications of this variability have yet to be deciphered. Regularly packed actin assemblies such as sheets, tubes or ribbons may ultimately yield structural relationships to in vivo relevant actin oligomers such as, for example, the "lower dimer".