Poly(C)-binding proteins (PCBPs) constitute a family of nucleic acid-binding proteins that play important roles in a wide spectrum of regulatory mechanisms. The diverse functions of PCBPs are dependent on the ability of the PCBPs to recognize poly(C) sequences with high affinity and specificity. PCBPs contain three copies of KH (hnRNP K homology) domains, which are responsible for binding nucleic acids. We have determined the NMR structure of the first KH domain (KH1) from PCBP2. The PCBP2 KH1 domain adopts a structure with three alpha-helices packed against one side of a three-stranded antiparallel beta-sheet. Specific binding of PCBP2 KH1 to a number of poly(C) RNA and DNA sequences, including the C-rich strand of the human telomeric DNA repeat, the RNA template region of human telomerase, and regulatory recognition motifs in the poliovirus-1 5'-untranslated region, was established by monitoring chemical shift changes in protein (15)N-HSQC spectra. The nucleic acid binding groove was further mapped by chemical shift perturbation upon binding to a six-nucleotide human telomeric DNA. The binding groove is an alpha/beta platform formed by the juxtaposition of two alpha-helices, one beta-strand, and two flanking loops. Whereas there is a groove in common with all of the DNA and RNA binders with a hydrophobic floor accommodating a three-residue stretch of C residues, nuances in recognizing flanking residues are provided by hydrogen bonding partners in the KH domain. Specific interactions of PCBP2 KH1 with telomeric DNA and telomerase RNA suggest that PCBPs may participate in mechanisms involved in the regulation of telomere/telomerase functions.