An archimedian spiral: the basal disk of the Wolinella flagellar motor.

The motor that powers the rotation of the bacterial flagellum reaches through both membranes into the cytoplasm of Gram-negative bacteria. The flagellum is connected by a flexible link (hook) to the motor axis, which passes through the center of a structure called the basal disk. The basal disk functions with the L-P ring complex as a bushing, enabling the rotation of the motor in the cell wall. The protein subunits of the basal disk of Wolinella succinogenes form an Archimedian spiral. The polymerization of subunits from a nucleation point at the motor in the form of a spiral allows constant growth of the basal disk. The disk is thought to provide a reinforcement at the flagellar insertion at the cell pole and to disperse forces that are generated by the momentum of the flagellar rotation.