2009 Volume 78 Issue 1 Pages 1_39-1_51
BmTRN-1, an RNA-binding protein homologous to mammalian TIA-1/TIAR, which possesses three RNA recognition motifs (RRMs) followed by a C-terminal auxiliary domain was shown to be involved in several mechanisms of RNA metabolism including the regulation of transcripts in the cells of the silkworm, Bombyx mori. The present gene analysis revealed that, in addition to previously reported mRNA, another isoform of mRNA with an alternatively spliced exon coding for an additional 14 amino acids in the auxiliary domain, was transcribed. Reporter protein production from the introduced plasmid was shown to be inhibited in the cultured cells by overexpressing both GFP-fused isoforms of BmTRN-1 with significant poly(U)-binding activity, but not by overexpressing the truncated mutant, such as RRM1-3 and the auxiliary domain. This indicates that the entire sequence of BmTRN-1 including both RRM1-3 and the auxiliary domain is necessary for the regulation of transcripts. Furthermore, analyses of the subcellular distribution of BmTRN-1 indicated that full-length BmTRN-1 is a shuttling protein present in both the nucleus and cytoplasm. However, BmTRN-1 was strongly distributed in the nucleus, especially when the cells were infected by the baculovirus, Autographa californica nucleopolyhedrovirus (AcNPV). Production of the glycoprotein of AcNPV, in the cytoplasm of infected cells, was clearly inhibited by the strong presence of both the full-length BmTRN-1 isoforms. In addition, the domains of BmTRN-1 related to its nuclear export and nuclear accumulation during viral infection were found to be RRM2 and RRM3, respectively. Thus, these findings indicate that BmTRN-1 with the domain responsible for its nuclear accumulation caused by the viral infection, was possibly related to the cellular function of eliminating baculovirus transcripts in the nucleus.
