Actin carries out many of its cellular functions through its filamentous form; thus, understanding the detailed structure of
actin filaments is an essential step in achieving a mechanistic understanding of
actin function. The
acrosomal bundle in the Limulus
sperm has been shown to be a quasi-crystalline array with an asymmetric unit composed of a filament with 14 actin-scruin pairs. The bundle in its true discharge state penetrates the jelly coat of the egg. Our previous
electron crystallographic reconstruction demonstrated that the
actin filament cross-linked by scruin in this
acrosomal bundle state deviates significantly from a perfect
F-actin helix. In that study, the
tertiary structure of each of the 14
actin protomers in the asymmetric unit of the bundle filament was assumed to be constant. In the current study, an
actin filament atomic model in the
acrosomal bundle has been refined by combining rigid-body docking with multiple
actin crystal structures from the
Protein Data Bank and constrained
energy minimization. Our observation demonstrates that
actin protomers adopt different tertiary conformations when they form an
actin filament in the bundle. The scruin and bundle packing forces appear to influence the tertiary and quaternary conformations of
actin in the filament of this biologically active bundle.
