Defining Structural Domains of an Intrinsically Disordered Protein: Sic1, the Cyclin-Dependent Kinase Inhibitor of Saccharomyces cerevisiae

The cyclin-dependent kinase inhibitor Sic1 is an intrinsically disordered protein (IDP) involved in cell-cycle regulation in the yeast Saccharomyces cerevisiae. Notwithstanding many studies on its biological function, structural characterization has been attempted only recently, fostering the development of production and purification protocols suitable to yield large amounts of this weakly expressed protein. In this study, we describe the identification of protein domains by the heterologous expression, purification, and characterization of Sic1-derived fragment. Four C-terminal fragments (Sic1(C-ter)) were produced based on functional studies and limited-proteolysis results. The N-terminal fragment (Sic1(1-186)) was complementary to the most stable C-terminal fragments (Sic1(Delta 186)). Both Sic1(1-186) and Sic1(C-ter) fragments were, in general, less susceptible to spontaneous proteolysis than the full-length protein. The boundaries of the C-terminal fragments turned out to be crucial for integrity of the recombinant proteins and required two rounds of design and production. Sic1 fragments were purified by a simple procedure, based on their resistance to heat treatment, at the amount and purity required for structural characterization. Circular dichroism (CD) measurements and nuclear magnetic resonance (NMR) spectra of N- and C-terminal fragments confirm their disordered nature but reveal minor structural differences that may reflect their distinct functional roles.