Structure and mechanics of the spasmoneme, a biological spring within the protozoan Vorticella convallaria

Abstract

Molecular springs have recently emerged as the basis for the fastest and most powerful movements at the cellular level in biology. The spasmoneme of the protozoan, Vorticella convallaria, is a model molecular spring, relying on energy stored in protein interactions to power contraction over a few hundred micrometers in a few milliseconds. While basic characteristics of Vorticella contraction are known, the underlying biochemical mechanism is unclear. The studies outlined here define and measure key parameters of spasmoneme performance which enable discrimination between proposed movement schemes and identification of new model parameters. Recent work has classified the spasmoneme as a power-limited machine (Upadhyaya, Baraban et al. 2007), where increases in viscous load correspond to decreases in velocity; in this work the maximum load at minimum velocity (the stall force), is measured. Work done by the stalk in contraction is shown to be dependent on its fractional change in length. This energy dependence arises from the basic underlying mechanism, and a major goal of this thesis was to characterize that mechanism by imaging the underlying structure. In the case of the Vorticella spasmoneme, imaging methods like birefringence imaging and electron microscopy, which do not require preexisting knowledge of protein identity, are particularly helpful.

(cont.) High-speed measurements of live Vorticella movements show the persistence of birefringence throughout the contraction-extension cycle. Orientation-independent measurements taken with an LC Pol-Scope show strong birefringence with slow axis parallel to the stalk long axis in both the extended and contracted states. Quantification of textural differences between the two states reveals slight structural disordering upon contraction. Transmission electron micrographs show a correlation between nanometer-scale filaments and the distribution of birefringence within the spasmoneme. As a whole these measurements indicate that any model of the contractile mechanism should consider the interactions of filamentous proteins at high concentrations which lead to longitudinal microscopic alignment in both the extended and contracted states. Implications of a proposed model are considered in the context of how they may be tested in vitro with purified constituent and homologous recombinant proteins, and how they can inform the development of biomimetic, nanoscale actuators.