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 int egrated structural approach which combines intermediate resolution ele ctron 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 t he actin subunits within F-actin filaments, the arrangement of actin m onomers within the crystalline tubes involves antiparallel packing int o dimers with p2 symmetry. Within the crystalline tubes, the actin mon omers are oriented so that the filament axis runs parallel with the sh eet plane and the intersubunit contacts in this direction are similar to those existing along the two long-pitch helical strands of the F-ac tin filament. The other intersubunit contacts within the crystalline t ubes 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 deciph ered. Regularly packed actin assemblies such as sheets, tubes or ribbo ns may ultimately yield structural relationships to in vivo relevant a ctin oligomers such as, for example, the ''lower dimer''. (C) 1998 Aca demic Press Limited.