The N-terminal domain of the Drosophila histone mRNA binding protein, SLBP, is intrinsically disordered with nascent helical structure

Stem-loop binding protein (SLBP) is a 31 kDa protein that is central to the regulation of histone mRNAs and is highly conserved in metazoans. In vertebrates, the N-terminal domain of SLBP has sequence determinants necessary for histone mRNA translation, SLBP degradation, cyclin binding, and histone mRNA import. We have used high-resolution NMR spectroscopy and circular dichroism to characterize the structural and dynamic features of this domain of SLBP from Drosophila (dSLBP). We report that the N-terminal domain of dSLBP is stably unfolded but has nascent helical structure at physiological pH and native-like solution conditions. The conformational and dynamic properties of the isolated domain are mimicked in a longer 175-residue region of the N-terminus, as well as in the full-length protein. Complete resonance assignments, secondary structure propensity, and motional properties of a 91-residue N-terminal domain (G17-K108) of dSLBP are reported here. The deviation of ¹H ^α, ¹³C^α, and ¹³Cβ chemical shifts from random coil reveals that there are four regions between residues I28-A45, S50-L57, S66-G75, and F91-N96 that have helical propensity. These regions also have small but positive heteronuclear NOEs, interresidue d_NN NOEs, and small but significant protection from solvent exchange. However the lack of medium- and long-range NOEs in 3D ¹⁵N- and ¹³C-edited spectra, fast amide proton exchange rates (all greater than 1 s^-1), and long ¹⁵N relaxation (T₁, T₂) times suggest that the domain from dSLBP does not adopt a well-defined tertiary fold. The backbone residual dipolar couplings (RDCs) for this domain are small and lie close to 0 Hz (±2 Hz) for most residues with no well-defined periodicity. The implications of this unfolded state for the function of dSLBP in regulating histone metabolism are discussed.