Single-step labeling of microtubule biomolecules using CdTe quantum dots

Introduction Kinesin is a naturally occurring protein capable of cargo transport upon interaction with a cytoplasmic system of fibers, known as microtubules. Hydrolysis of ATP propels kinesin along microtubules, being... [ view full abstract ]

Introduction

Kinesin is a naturally occurring protein capable of cargo transport upon interaction with a cytoplasmic system of fibers, known as microtubules. Hydrolysis of ATP propels kinesin along microtubules, being therefore a promising tool in the development of synthetic nano transport machines. One of the challenges to study such system is how to observe the motility of either kinesin or microtubule.

Here, we report the use of mercaptopropionic acid (MPA)–capped cadmium telluride (CdTe) quantum dots (QDs) as a fluorescent labeling agent to microtubule biomolecules.

Method

Microtubules were polymerized from commercially available tubulin protein (Cytoskeleton Inc.).

MPA-capped CdTe QDs were synthesized using a fixed molar ratio of 1:7:0.25 (Cd⁺²:MPA:Te). QDs were characterized microscopically and spectroscopically and the binding affinity of QDs to microtubules was studied via the surface plasmon resonance (SPR) technique.

Results and Discussion

Figure 1 shows the TEM image of synthesized CdTe QDs, with average diameters of 3 nm; while figure 2 shows the fluorescence spectra, as well as the UV-vis spectra (inset) for CdTe QDs in water, exhibiting high fluorescence with emission maxima at 545 nm.

Figure 3 displays obtained SPR data for immobilization of microtubule on the substrate’s surface followed by injection of CdTe QDs over the microtubule-coated surface. Insets show only the binding interaction between QDs and the microtubule-bound surface.

The measured SPR angle (Δθ) continues to rise throughout the flow of CdTe QDs over immobilized microtubules, suggesting strong binding between biomolecule and QDs.

QDs are believed to adsorb to microtubule’s surface through a specific interaction with the histidine amino acids present in the microtubule’s surface. The imidazole-nitrogen donor of histidine is an important binding site for transition metal ions in biological system.

Figure 4 shows fluorescence image for a microtubule functionalized with CdTe QDs. As can be seen, QDs are able to not only bind to microtubules in vitro, but also to fluorescently label the biomolecule in the green spectral region.

To the best of author’s knowledge, no other study to date showed the use of QDs to directly fluorescently label microtubules.

This work was supported by FAPESP grants number 2015/01271-3 and 2013/24725-4