W. J. Chazin

Human pre‐mRNA processing protein 40 homolog A (hPrp40A) is a splicing factor that interacts with the Huntington's disease protein huntingtin (Htt). Evidence has accumulated that both Htt and hPrp40A are modulated by the intracellular Ca 2+ sensor calmodulin (CaM). Here we report characterization of the interaction of human CM with the third FF domain (FF 3 ) of hPrp40A using calorimetric, fluorescence and structural approaches. Homology modeling, differential scanning calorimetry and small angle X‐ray scattering (SAXS) data show FF 3 forms a folded globular domain. CaM was found to bind FF 3 in a Ca 2+ ‐dependent manner with a 1:1 stoichiometry and a dissociation constant ( K d ) of 25 ± 3 μM at 25°C. NMR studies showed that both domains of CaM are engaged in binding and SAXS analysis of the FF 3 ‐CaM complex revealed CaM occupies an extended configuration. Analysis of the FF 3 sequence showed that the anchors for CaM binding must be buried in its hydrophobic core, suggesting that binding to CaM requires unfolding of FF 3 . Trp anchors were proposed based on sequence analysis and confirmed by intrinsic Trp fluorescence of FF 3 upon binding of CaM and substantial reductions in affinity for Trp‐Ala FF 3 mutants. The consensus model of the complex showed that binding to CaM binding occurs to an extended, non‐globular state of the FF 3 , consistent with coupling to transient unfolding of the domain. The implications of these results are discussed in the context of the complex interplay of Ca 2+ signaling and Ca 2+ sensor proteins in modulating Prp40A‐Htt function.