“…It has long been noted that certain RNA-binding proteins can be characterized, grouped, and purified on the basis of their binding to nucleic acid homopolymers (Swanson & Dreyfuss, 1988)+ A defined set of RNAbinding proteins is characterized by high affinity and sequence-specific interaction with poly(C)+ These poly(C)-binding proteins (PCBPs) comprise two subsets in mammalian cells; hnRNPs K/J (Matunis et al+, 1992) and the aCP proteins (a-complex proteins)+ The exact structural and genetic relationship of hnRNP K and J remains to be fully defined+ The aCPs are encoded at four dispersed loci, with additional isoforms generated via alternative splicing (Makeyev & Liebhaber, 2000)+ The PCBPs studied in greatest detail are hnRNP K, aCP-1, and aCP-2+ The latter two proteins are alternatively referred to as PCBP1 and PCBP2 or hnRNP-E1 and hnRNP-E2 (Kiledjian et al+, 1995;Leffers et al+, 1995)+ Recent studies, summarized in this review, reveal that the PCBPs are involved in a remarkable array of biological processes+ Members of this protein family are linked to mRNA stabilization (Weiss & Liebhaber, 1994Holcik & Liebhaber, 1997), translational silencing (Ostareck et al+, 1997;Collier et al+, 1998), and translational enhancement (Blyn et al+, 1997;Gamarnik & Andino, 1997)+ These proteins also appear to be involved as determinants of transcriptional controls (Michelotti et al+, 1996;Tomonaga & Levens, 1996) and apoptotic pathways (Charroux et al+, 1999;Zhu & Chen, 2000) and constitute candidate structural components of recombination complexes within retrotransposon long terminal repeats (Goller et al+, 1994) and in telomere complexes (Lacroix et al+, 2000)+ The common denominator of PCBP activities appears to reflect binding to C-rich single-strand motifs+ Determining how such interactions might factor into this broad array of diverse biologic functions constitutes a major challenge to current research efforts+ Initial studies of PCBPs have been previously reviewed Ostareck-Lederer et al+, 1998)+ This article will focus with particular attention on three areas of recent interest: (1) identification and characterization of novel PCBP isoforms and posttranslational modifications that may underlie their functional diversity; (2) new insights into the evolutionary history of the PCBPs that may shed light on their conserved structure-function relationships, and (3) the expanding spectrum of PCBP functions in transcriptional and posttranscriptional controls+…”