| Plant genomes have allowed the expansion of many types of mobile genetic elements. LTR retrotransposons are a subclass of mobile genetic elements that replicate using an RNA intermediate. The Sireviruses are one of three genera in the Ty1/copia class of LTR retrotransposons. The name 'Sireviruses' comes from the founding member (soybean SIRE1) and the fact that retrotransposons, although not known to be infectious, have many traits in common with viruses.; Besides being phylogenetically distinct, the Sireviruses have features that set them apart from other LTR retrotransposons. Different Sirevirus members show great variability in their genomic structures and the translational tricks they use to express encoded proteins. We have shown that the SIRE1 elements of soybean use stop codon suppression to express their Env-like protein. Some monocot members of the Sireviruses may use a bypass mechanism to translate Pol.; Another feature of the Sireviruses is that most carry additional coding information in the form of an open reading frame (ORF) referred to as an env-like ORF, and all have encoded extra coding information in their gag gene. The env-like ORF has caused speculation that these elements are plant retroviruses, although no experimental evidence has determined this to be true. However, using a yeast two-hybrid screen, we have discovered an interaction between multiple Sirevirus Gags and a family of related host cell proteins referred to as dynein light chain LC8 and LC6. The LC8 and LC6 proteins are highly conserved in eukaryotes and are known components of the dynein and myosin-V motors. LC8 can bind cargo (cell proteins or virus particles) to allow movement along the cytoskeleton. Thus, the interaction of the Sirevirus Gags and LC8 or LC6 may be to move the Sirevirus VLPs or transposition intermediates within a cell (for example, from cytoplasmic to nuclear compartments). If true, this would not only represent the first example of a movement mechanism for any retrotransposon, but it also illustrates how plant retrotransposons and plant viruses use similar mechanisms to achieve a common goal. In addition, an initial characterization of the expression and localization of the Arabidopsis thaliana LC8/LC6 gene family was completed. |
