Role of polypyrimidine tract binding protein in the function of the hepatitis B virus posttranscriptional regulatory element

The hepatitis B virus posttranscriptional regulatory element (PRE) is an RN A element that increases the expression of unspliced mRNAs, apparently by f acilitating their export from the nucleus. We have identified a cellular pr otein that binds to the PRE as the polypyrimidine tract binding protein (PT B), which shuttles rapidly between the nucleus and the cytoplasm. Mutants o f the PRE with mutations in PTB binding sites show markedly decreased activ ity, while cells that stably overexpress PTB show increased PRE-dependent g ene expression. Export of PTB from the nucleus, like PRE function, is block ed by a mutant form of Ran binding protein 1 but not by leptomycin B. There fore, PTB is important for PRE activity and appears to function as an expor t factor for PRE-containing mRNAs.