Temperature Control of Biotin Binding and Release with A Streptavidin-Poly(N-isopropylacrylamide) Site-Specific Conjugate

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Abstract
The many laboratory and diagnostic applications utilizing streptavidin as a molecular adaptor rely on its high affinity and essentially irreversible interaction with biotin. However, there are many situations where recovery of the biotinylated molecules is desirable. We have previously shown that poly(N-isopropylacrylamide) (PNIPAAm), a temperature-sensitive polymer, can reversibly block biotin association as the polymer's conformation changes at its lower critical solution temperature (LCST). Here, we have constructed a streptavidin-PNIPAAm conjugate which is able to bind biotin at room temperature or lower and release bound biotin at 37 degrees C. The conjugate can repeatedly bind and release biotin as temperature is cycled through the LCST. A genetically engineered streptavidin mutant, E116C, which has only one cysteine residue, was conjugated site specifically via the sulfhydryl groups with a PNIPAAm that has pendent sulfhydryl-reactive vinyl sulfone groups. The conjugation site is near the tryptophan 120 residue, which forms a van der Waals contact with biotin that is important in generating the large binding free energy. The temperature-induced conformational change of the polymer at position 116 may lead to structural changes in the region of tryptophan 120 that are responsible for the reversible binding between biotin and the conjugated streptavidin.