Phloem RNA-binding proteins: Discovery and characterization of a phloem polypyrimidine tract-binding protein and the implications for regulation of systemic RNA movement

The ultimate pattern of gene expression is controlled by a complex network of cotranscriptional, post-transcriptional and translational regulation. RNA-binding proteins (RBPs) are part of a changing ensemble of proteins that associate with transcripts to form a ribonucleoprotein (RNP) complex and mediate progression of pre-mRNA and mRNA from transcription through translation to decay. There is increasing evidence that plants use non-cell autonomous movement of RNPs to coordinate regulation of developmental and physiological processes. Plasmodesmata, intercellular organelles, allow trafficking of proteins and RNPs between cells and through connections to the phloem CC-SE complex they create a systemic pathway that may allow inter-organ movement of information molecules. Systemic RNA signaling is consistent with the long-distance transmission of RNA interference and the ability of translocated aberrant transcripts to alter developmental phenotypes. Certain transcripts localized to phloem sap undergo selective unloading in apical tissues, indicating that long-distance movement of transcripts may be regulated. In animal cells, intracellular localization of RNPs is mediated by RBPs. Likewise, systemic transport of RNA in plants may be mediated by RBPs, however few endogenous RBPs localized to phloem sap have been identified. In order to identify components that mediate systemic signaling a screen of pumpkin phloem sap for RBPs was conducted. This lead to the identified a 50-kD phloem-localized protein, Cucurbita maxima RNA-binding protein, 50kD (CmRBP50) with sequence similarity to animal polypyrimidine tract-binding proteins (PTBs). PTBs are multifunctional RBPs involved in localization and processing of RNA, including nuclear export of mRNA, alternative splicing of pre-mRNA, mRNA stabilization, regulation of cap-independent translation, and poly(A) site cleavage. Heterograft analysis indicated that CmRBP50 is translocated long-distance in the phloem. CmRBP50 is characterized with respect to conserved PTB-like activity, RNA-binding properties, and interaction with other potential components of long-distance signaling. The possible role of CmRBP50 in selective trafficking of transcripts, and the potential of CmRBP50 for multi-functional activity in long-distance signaling, is discussed.