Abstract
Polypyrimidine tract-binding (PTB) proteins are a family of RNA-binding proteins that function in a wide range of RNA metabolic processes by binding to motifs rich in uracils and cytosines. A PTB protein of pumpkin was identified as the core protein of an RNA-protein complex that trafficks RNA. The biological function of the PTB-RNA complex, however, has not been demonstrated. In potato, six PTB proteins have been identified, and two, designated StPTB1 and StPTB6, are similar to the phloem-mobile pumpkin type. RNA binding assays confirmed the interaction of StPTB1 and StPTB6 with discrete pyrimidine-rich sequences of the 3'-untranslated regions of the phloem-mobile mRNA, StBEL5. The promoter of StPTB1 was active in companion cells of phloem in both stem and petioles. Expression of both types was evident in phloem cells of roots and in stolons during tuber formation. RNA accumulation of both PTB proteins was induced by short days in leaves in correlation with enhanced accumulation of StBEL5 RNA. StPTB suppression lines exhibited reduced tuber yields and decreased StBEL5 RNA accumulation, whereas StPTB overexpression lines displayed an increase in tuber production correlated with the enhanced production in stolons of steady-state levels of StBEL5 transcripts and RNA of key tuber identity genes. In StPTB overexpression lines, both the stability and long-distance transport of StBEL5 transcripts were enhanced, whereas in suppression lines stability and transport decreased. Using a transgenic approach, it is shown that the StPTB family of RNA-binding proteins regulate specific stages of development through an interaction with phloem-mobile transcripts of StBEL5.
Highlights
The polypyrimidine tract-binding (PTB) family of proteins represent a multifaceted group of proteins that binds numerous mRNAs and have been implicated in a wide range of RNA metabolic processes including stability (Xu and Hecht, 2007), splicing regulation (Valcarcel and Gebauer, 1997; Xue et al, 2009), intracellular localization (Kuwahata et al, 2007), translation repression (Karakasiliotis et al, 2010), and control of long-distance transport (Ham et al, 2009)
The source of the mobile transgenic StBEL5 RNA is the same for all stocks analysed. These results suggest that steady-state levels of StBEL5 in both stolon tips and roots are due to the stability and movement of its mRNA both mediated by an interaction with StPTB proteins
Among the six Polypyrimidine tract-binding (PTB) proteins of potato, StPTB1 and StPTB2 are most similar in sequence to the phloem-mobile PTB protein of pumpkin, CmRBP50 (Ham et al, 2009), and to AtPTB3 of Arabidopsis (Rühl et al, 2012)
Summary
The polypyrimidine tract-binding (PTB) family of proteins represent a multifaceted group of proteins that binds numerous mRNAs and have been implicated in a wide range of RNA metabolic processes including stability (Xu and Hecht, 2007), splicing regulation (Valcarcel and Gebauer, 1997; Xue et al, 2009), intracellular localization (Kuwahata et al, 2007), translation repression (Karakasiliotis et al, 2010), and control of long-distance transport (Ham et al, 2009). The structure of a PTB protein is uniquely adapted to these multiple functions. They generally contain four RNA-recognition motifs (RRMs) of ~90 amino acids connected by linker regions and designated RRM1, RRM2, RRM3, and RRM4. Each RRM is formed by 4–5 β-sheets and contains 6–8 conserved amino acids, designated RNP1 and RNP2, that interact with CU (cytosine uracil) motifs, ranging from three to five nucleotides in length (Oberstrass et al, 2005; Auweter and Allain, 2008). Structural analysis has revealed that RRM1 and RRM2 are linear and function independently, whereas RRM3 and RRM4 act in a tandem, compact complex that functions as an open-faced clamp on closely spaced polypyrimidine tract motifs (Oberstrass et al, 2005)
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