Characterization of Mesoscopic Distortions within an Actin Filament in Vitro

Kuniyuki Hatori and Tomoyuki Kikuchi

Department of Bio-System Engineering, Yamagata University


We have already investigated fluctuations of fluorescently labeled actin filaments sliding on Heavy Meromyosin molecules (HMM) under a fluorescence microscope by the detection of its positions in a plane with nano-meter accuracy.
Here, we attempted the characterization of the two kinds of fluctuations occurring on a certain part within an actin filament as mesoscopic distortions: one was the fluctuations of displacement (FD), the other was the fluctuations of fluorescent intensity profile which was referred to its maximum intensity (FI) and standard deviation as distribution (FS). In order to trace motion of a certain part within a filament, we employed speckled actin filaments, which were prepared with mixture both rhodamine-phalloidin labeled segments and non-labeled segments. Thermal motions of actin filaments were restricted on a glass surface by aid of biotin-avidin bonds, but semi-flexibility was preserved because of adjustment for the binding strength. FD, FI and FS in the semi-flexible segment were measured at every interval of 1/30 sec.
When the auto-correlation functions of fluctuations in the presence of ATP were examined, the correlation intensities for FI and FD were decayed with the correlation time of 0.1 sec and 0.05 sec respectively. The cross-correlation between FI and FS negatively strengthened as delay-time was zero. This result may indicate that fluorescent intensity we measured fluctuated due to density change in a fluorescent segment associated with local distortions.