Heterogeneous nuclear ribonucleoprotein (hnRNP) implicated in mRNA processing mechanisms. Component of the CRD-mediated complex that promotes MYC mRNA stability. Isoform 1 and isoform 2 are associated in vitro with pre-mRNA, splicing intermediates and mature mRNA protein complexes. Isoform 1 binds to apoB mRNA AU-rich sequences (By similarity). Isoform 1 is part of the APOB mRNA editosome complex and may modulate the postranscriptional C to U RNA-editing of the APOB mRNA through either by binding to A1CF (APOBEC1 complementation factor), to APOBEC1 or to RNA itself (By similarity). May be involved in translationally coupled mRNA turnover. Implicated with other RNA-binding proteins in the cytoplasmic deadenylation/translational and decay interplay of the FOS mRNA mediated by the major coding-region determinant of instability (mCRD) domain (By similarity). Interacts in vitro preferentially with poly(A) and poly(U) RNA sequences. Isoform 2 may be involved in cytoplasmic vesicle-based mRNA transport through interaction with synaptotagmins. Identified in the spliceosome C complex. Component of the coding region determinant (CRD)-mediated complex, composed of DHX9, HNRNPU, IGF2BP1, SYNCRIP and YBX1. Identified in a mRNP complex, at least composed of DHX9, DDX3X, ELAVL1, HNRNPU, IGF2BP1, ILF3, PABPC1, PCBP2, PTBP2, STAU1, STAU2, SYNCRIP and YBX1. Identified in a mRNP granule complex, at least composed of ACTB, ACTN4, DHX9, ERG, HNRNPA1, HNRNPA2B1, HNRNPAB, HNRNPD, HNRNPL, HNRNPR, HNRNPU, HSPA1, HSPA8, IGF2BP1, ILF2, ILF3, NCBP1, NCL, PABPC1, PABPC4, PABPN1, RPLP0, RPS3, RPS3A, RPS4X, RPS8, RPS9, SYNCRIP, YBX1 and untranslated mRNAs. Interacts with GTPBP1 (By similarity). Isoform 1 is a component of the APOB mRNA editosome complex. Isoform 1 interacts with APOBEC1 and A1CF. Part of a complex associated with the FOS mCRD domain and consisting of PABPC1, PAIP1, CSDE1/UNR, HNRPD and SYNCRIP. Isoform 2 interacts with HNRPR. Interacts with POLR2A hyperphosphorylated C-terminal domain. Interacts with HABP4 (By similarity). Identified in a histone pre-mRNA complex, at least composed of ERI1, LSM11, SLBP, SNRPB, SYNCRIP and YBX1. Isoform 1 and isoform 2 interact with SMN. Isoform 2 interacts through its C-terminal domain with SYT7, SYT8 and SYT9. The non-phosphorylated and phosphorylated forms are colocalized with PAIP1 in polysomes. Localized in cytoplasmic mRNP granules containing untranslated mRNAs (By similarity). Isoforms 1 and 2 are expressed predominantly in the nucleoplasm. According to PubMed:10734137, isoform 2 is predominantly found in cytoplasm. The tyrosine phosphorylated form bound to RNA is found in microsomes. Ubiquitous. Detected in heart, brain, spleen, lung, liver, skeletal muscle, adipocytes, kidney and testis. Expressed in spinal cord at 14 dpc and onwards. The domain containing eight Arg-Gly-Gly repeats (RGG/RXR-box) may be involved in RNA-binding and protein-protein interactions. It is methylated by PRMT1, and essential for nuclear localization (By similarity). Phosphorylated on tyrosine. The membrane-bound form found in microsomes is phosphorylated in vitro by insulin receptor tyrosine kinase (INSR). Phosphorylation is inhibited upon binding to RNA, whereas the cytoplasmic form is poorly phosphorylated. May be due to a competing donor splice site and to an exon inclusion.