hnRNP A1: Proteins and Enzymes

RNA polymerase II transcripts in the nucleus are in complex with several proteins called heterogeneous nuclear ribonucleoproteins (hnRNPs). These proteins are important in biological activities such as transcription, pre-mRNA processing, cytoplasmic mRNA translation and turnover. hnRNPs can be isolated either by immunoprecipitation or by sucrose gradient fractionation of cell extracts.1, 3, 4 Isolated hnRNPs consist of protein groups named A to U and many of these protein groups consist of more than one isoform. The major steady-state proteins of the isolated hnRNP complex are A1, A2, B1, B2, C1, and C2, with a range of molecular weight starting with 34 kDa up to 43 kDa.1, 3, 4 hnRNP-A1 is important in pre-mRNA processing and in mRNA export from the nucleus. The protein binds to its RNA target through a consensus RNA-binding site UAGGGU. The protein contains a 38-amino acid domain called M9, which is important for the interaction with the transportin protein, and therefore, for its import and export from the nucleus. RanGTP mediates dissociation of hnRNP-A1 from transportin. hnRNP-A1 is ubiquitously expressed with a higher expression in proliferating and/or transformed cells than in differentiated tissues.5