Huntingtin interacting protein HYPK is intrinsically unstructured

To characterize HYPK, originally identified as a novel huntingtin (Htt) interacting partner by yeast two hybrid assay, we used various biophysical and biochemical techniques. The molecular weight of the protein, determined by gel electrophoresis, was found to be about 1.3-folds (∼22 kDa) higher than that obtained from mass spectrometric analysis (16.9 kDa). In size exclusion chromatography experiment, HYPK was eluted in three fractions, the hydrodynamic radii for which were calculated to be ∼1.5-folds (23.06 Å) higher than that expected for globular proteins of equivalent mass (17.3 Å). The protein exhibited predominantly (63%) random coil characteristics in circular dichroism spectroscopy and was highly sensitive to limited proteolysis by trypsin and papain, indicating absence of any specific domain. Experimental evidences with theoretical analyses of amino acids composition of HYPK and comparison with available published data predicts that HYPK is an intrinsically unstructured protein (IUP) with premolten globule like conformation. In presence of increasing concentration of Ca²⁺, HYPK showed conformational alterations as well as concomitant reduction of hydrodynamic radius. Even though any link between the natively unfolded nature of HYPK, its conformational sensitivity towards Ca²⁺ and interaction with Htt is yet to be established, its possible involvement in Huntington's disease pathogenesis is discussed.