AbstractFibroblast growth factor 2 (FGF-2) has a dual role as a classical extracellular signaling protein and as an intracellular factor. Isoforms of FGF-2, resulting from alternatively used start codons on one mRNA species, locate differentially to nuclear compartments. In this study we aimed to analyze functions of intracellular FGF-2 by identification of interacting proteins. We identified the 66-kDa subunit of splicing factor 3a (SF3a66) as a binding partner in a yeast two-hybrid screen and confirmed this interaction by pull-down assays. The splicing factor interacted with the 18-kDa (FGF-218) and with the 23-kDa (FGF-223) isoforms, indicating an interaction with a domain common to both isoforms. Moreover, FGF-2 interacted with the C-terminus of SF3a66, a sequence that has not previously been assigned a functional role. In a functional neurite outgrowth assay, SF3a66 enhanced neurite lengths similar to FGF-218. We have previously identified the spliceosomal assembly factor survival of motoneuron (SMN) protein as a protein interacting specifically with the FGF-223isoform [Claus et al., J. Biol. Chem.278(2003), 479–485]. The identification of two FGF-2 interacting proteins from the same biochemical pathway suggests a novel intranuclear role of FGF-2.