"There have been some reports where proteins are used as templates for synthesizing or arraying nanoparticles," Kazushi Kinbara of the University of Tokyo told nanotechweb.org. "Our results are the first example of bioresponsive semiconductor nanoparticles, where the unique biological function of chaperonin proteins is integrated with the property of nanoparticles."
Kinbara and colleagues used the chaperonin protein GroEL from the bacteria Escherichia coli and the protein T.th cpn from Thermus thermophilus. GroEL is 14.6 nm high, with a wall thickness of 4.6 nm and a central cylindrical cavity that is 4.5 nm in diameter. T.th cpn has a similar cavity size and wall thickness to GroEL, but also contains a capping protein assembly on either side of its cavity.
Normally, these chaperonins encapsulate denatured proteins inside their central cavities and help them to refold. After refolding, the presence of adenosine triphosphate (ATP) causes the chaperonin to change its shape and release the protein. In this study, the team encapsulated CdS nanoparticles with a diameter of 2-4 nm inside the chaperonins. Transmission electron microscope studies indicated that about 75% of the T.th cpn molecules took up a nanoparticle, with a single nanoparticle entering each cavity. The chaperonin/nanoparticle complexes were stable up to about 80°C, tolerant to electrolytes and electronically dormant.
To release the nanoparticles, the scientists introduced ATP in the presence of magnesium and potassium ions. As expected, the chaperonin proteins bound with the ATP, underwent a conformational change and freed the nanoparticles.
"We consider that the chaperonin/CdS nanoparticle complex can be applied in detecting ATP, which is the most important energy source in the biological system," added Kinbara. "In addition, by connecting the chaperonin proteins in a one-dimensional array, we could fabricate nanowires that would be responsive to ATP. It would also be possible to use chaperonin proteins as drug carriers if we can introduce drugs instead of nanoparticles into the cavity."
Now the researchers say they can introduce functional groups at any location by using mutant chaperonin. "We would like to give additional functions to the chaperonin/nanoparticle complexes by chemical modification," added Kinbara.
The scientists reported their work in Nature.