This entry represents the RNA recognition motif 1 (RRM1) of polypyrimidine tract-binding protein 1 (PTBP1). PTBP1 (also known as PTB) is involved in numerous post-transcriptional steps in gene expression in both the nucleus and cytoplasm. It can act as a negative regulator of alternative splicing and as an activator of translation driven by IRESs (internal ribosome entry segments) []. It contains four RNA recognition motifs (RRM). 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 [, ].
This entry represents the RNA recognition motif 2 (RRM2) of polypyrimidine tract-binding protein 1 (PTBP1) and related proteins. Proteins containing this domain include PTBP1/2/3 (or PTB1/2/3) and heterogeneous nuclear ribonucleoprotein L-like (HNRNPLL) proteins. PTB can shuttle between nucleus and cytoplasm. It is a splicing repressor factor implicated in the control of alternative exon selection during mRNA processing of many different transcribed genes, including its own pre-mRNA. It is also involved in internal ribosome entry site (IRES)-mediated translation initiation. It may also be involved in the 3'-end processing, localization, and stability of several mRNAs. Two PTB homologues, PTBP2 and PTBP3, are generally expressed in mammalian tissues. PTBP2 is expressed at high levels in adult brain, muscle and testis, while PTBP3 is expressed preferentially in haematopoietic cells []. PTB and PTBP2 bind to the same RNA sequences and have similar effects on alternative splicing events. However, differential expression of PTB and PTBP2 can lead to the generation of alternatively spliced mRNAs []. During neuronal differentiation, MicroRNA miR-124 downregulates PTBP1 expression, which in turn leads to upregulation of PTBP2. Later in development, the expression of PTBP2 decreases and this leads to a second wave of alternative splicing changes characteristic of adult brain and essential for brain development []. PTBP3 may be involved in nonsense-mediated mRNA decay [].
This entry represents the RNA recognition motif 2 (RRM2) of polypyrimidine tract-binding protein 1 (PTB or hnRNP I), polypyrimidine tract-binding protein 2 (PTBP2 or nPTB), polypyrimidine tract-binding protein 3 (PTBP3 or Rod1), and similar proteins found in Metazoa.PTB can shuttle between nucleus and cytoplasm. It is a splicing repressor factor implicated in the control of alternative exon selection during mRNA processing of many different transcribed genes, including its own pre-mRNA. It is also involved in internal ribosome entry site (IRES)-mediated translation initiation. It may also be involved in the 3'-end processing, localization, and stability of several mRNAs. Two PTB homologues, PTBP2 and PTBP3, are generally expressed in mammalian tissues. PTBP2 is expressed at high levels in adult brain, muscle and testis, while PTBP3 is expressed preferentially in haematopoietic cells []. PTB and PTBP2 bind to the same RNA sequences and have similar effects on alternative splicing events. However, differential expression of PTB and PTBP2 can lead to the generation of alternatively spliced mRNAs []. During neuronal differentiation, MicroRNA miR-124 downregulates PTBP1 expression, which in turn leads to upregulation of PTBP2. Later in development, the expression of PTBP2 decreases and this leads to a second wave of alternative splicing changes characteristic of adult brain and essential for brain development []. PTBP3 may be involved in nonsense-mediated mRNA decay [].
