New insights into functional roles of the polypyrimidine tract-binding protein

Abstract

Polypyrimidine Tract Binding Protein (PTB) is an intensely studied RNA binding protein involved in several post-transcriptional regulatory events of gene expression. Initially described as a pre-mRNA splicing regulator, PTB is now widely accepted as a multifunctional protein shuttling between nucleus and cytoplasm. Accordingly, PTB can interact with selected RNA targets, structural elements and proteins. There is increasing evidence that PTB and its paralog PTBP2 play a major role as repressors of alternatively spliced exons, whose transcription is tissue-regulated. In addition to alternative splicing, PTB is involved in almost all steps of mRNA metabolism, including polyadenylation, mRNA stability and initiation of protein translation. Furthermore, it is well established that PTB recruitment in internal ribosome entry site (IRES) activates the translation of picornaviral and cellular proteins. Detailed studies of the structural properties of PTB have contributed to our understanding of the mechanism of RNA binding by RNA Recognition Motif (RRM) domains. In the present review, we will describe the structural properties of PTB, its paralogs and co-factors, the role in post-transcriptional regulation and actions in cell differentiation and pathogenesis. Defining the multifunctional roles of PTB will contribute to the understanding of key regulatory events in gene expression.

Figure 1

RNA binding proteins are involved in all steps of RNA biogenesis. Starting from pre-mRNA transcript synthesis, a variety of different RNA binding proteins are associated within the heterogeneous ribonucleoprotein complexes, participating in the processes which result in the formation of mature mRNA, including pre-mRNA splicing and addition of 3′ poly(A) tails in the nucleus and mRNA export through the nuclear pores. In the cytoplasm, they participate in localization, initiation of translation, and mRNA stability.

Figure 2

PTB RRM3 structure. (A) The PTB RRM3 structure was obtained with Swiss-PdbViewer (http://www.expasy.org/spdbv/); (B) Alignments of ribonucleoprotein (RNP) 2 and RNP1 sequences present in the PTB RRM3 with RRM containing proteins. The alignment was generated by ClustalW and manually optimized. PTB, polypryrimidine tract binding protein; nPTB, neuronal PTB; hnRNPA1, heterogeneous nuclear ribonucleoprotein type A1; LA, La protein; TAP, mRNA export factor Tap; PABP, Poly(A)-binding protein; CSTF64, cleavage stimulation factor 64; Y14, Y14 Magoh complex; HUD, Hu protein; SXL, Sex-Lethal; U1A, U2B, U1 snRNP proteins; CBP20, cap-binding protein 20; UPF3, up-frameshift protein 3; U2AF65, U2 snRNP auxiliary factor; RBM20, RNA binding motif 20.

Figure 3

Models of RNA-PTB interaction in exon splicing repression. (A) Repression by PTB looping out a branch-point adenosine; (B) PTB monomer looping-out an exon; (C) Multiple PTB molecules binding cooperatively around an alternative exon; (D) PTB cooperates with the co-factor Raver1 to loop out an alternative exon by binding to distant intronic pyrimidine tracts.

Figure 4

Relative ratio of PTB and nPTB expression during cell differentiation.