RNA recognition motif 4 (RRM4) found in vertebrate polypyrimidine tract-binding protein 1 (PTB)
This subgroup corresponds to the RRM4 of PTB, also known as 58 kDa RNA-binding protein PPTB-1 or heterogeneous nuclear ribonucleoprotein I (hnRNP I), an important negative regulator of alternative splicing in mammalian cells. PTB also functions at several other aspects of mRNA metabolism, including mRNA localization, stabilization, polyadenylation, and translation. PTB contains four RNA recognition motifs (RRM), also known as RBD (RNA binding domain) or RNP (ribonucleoprotein domain). RRM1 and RRM2 are independent from each other and separated by flexible linkers. By contrast, there is an unusual and conserved interdomain interaction between RRM3 and RRM4. It is widely held that only RRMs 3 and 4 are involved in RNA binding and RRM2 mediates PTB homodimer formation. However, new evidence shows that the RRMs 1 and 2 also contribute substantially to RNA binding. Moreover, PTB may not always dimerize to repress splicing. It is a monomer in solution.