Alternative splicing plays an important role in generating proteome diversity. The polypyrimidine tract–binding protein (PTB) is a key alternative splicing factor involved in exon repression. It has been proposed that PTB acts by looping out exons flanked by pyrimidine tracts. We present fluorescence, NMR, and in vivo splicing data in support of a role of PTB in inducing RNA loops. We show that the RNA recognition motifs (RRMs) 3 and 4 of PTB can bind two distant pyrimidine tracts and bring their 5′ and 3′ ends in close proximity, thus looping the RNA. Efficient looping requires an intervening sequence of 15 nucleotides or longer between the pyrimidine tracts. RRM3 and RRM4 bind the 5′ and the 3′ pyrimidine tracts, respectively, in a specific directionality and work synergistically for efficient splicing repression in vivo.

Significance: Activation of G protein-coupled receptors (GPCRs) by agonists is the first step of eukaryotic cellular signal transduction. Because GPCRs are expressed in almost all human tissues and play a key role in human physiology, they are the targets for more than 30% of pharmaceutical drugs. Binding of ligands on the extracellular surface of a GPCR induces a conformational change on the cytoplasmic surface, which is recognized by G proteins or other cellular effectors. Here we show that the β ₂ -adrenergic receptor, a prototypical GPCR, naturally fluctuates between inactive and active conformations, and that agonist or inverse agonist ligands modulate the conformational exchange kinetics in distinct ways, explaining their different pharmacological efficacies. These insights should assist in the design of improved GPCR-targeting drugs.