Jeri Brandom 1 , Byung-Chun Yoo 2 , Nien-Chen Huang 1 and William J. Lucas 1 .

There is increasing evidence that plants use non-cell autonomous movement of ribonucleoproteins (RNPs) to coordinate regulation of developmental and physiological processes (Haywood et al. , 2002). Systemic RNA signaling is consistent with the long-distance transmission of RNA interference and the ability of translocated aberrant transcripts to alter developmental phenotypes. In animals, RNA localization is a key determinant of developmental fate. RNA localization also occurs during embryogenesis and polarized cell growth in plants (Okita and Choi, 2002). Many RNA-binding proteins (RBPs) involved in cytoplasmic localization of RNA in animals have been identified and several of these, by sequence similarity, have apparent homologs in plants. However, little is known about proteins that may mediate long-distance RNA signaling in plants. Polypyrimidine tract-binding protein (PTB, also known as hnRNP I) is an RBP involved intracellular RNA localization in animals (Cote et al., 1999). It is a multi-functional RBP with four loosely conserved RNA-recognition motifs (RRM). PTB binds to C/U rich polypyrimidine tracts and can be purified from nuclear extracts with poly(U) affinity chromatography. In addition to RNA-localization, PTB plays an important role in the intracellular 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. PTB proteins have been identified in many animal systems and may be ubiquitous in metazoans. Sequence similarity indicates the presence of PTB homologs in several plant species. The Arabidopsis genome contains both PTB homologs and homologs of proteins that antagonize the activity of PTB in 3′-splice site selection. However, no plant PTB has yet been characterized.