Abstract
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 Pseudoviridae (Ty1/copia) are a family of LTR retrotransposons, and the Sireviruses are one of three genera in the Pseudoviridae. The Sireviruses have features that set them apart from classical retrotransposons. Different members of the Sireviruses show great variability in their genomic structures and the translational tricks they use to express their encoded proteins. For example, we have shown that the SIRE1 elements of soybean use stop codon suppression to express their Env-like protein. Secondly, some monocot members of the Sireviruses may use a bypass mechanism to translate Pol. Another notable 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 components of the dynein and myosin-V motors. LC8 can bind cargo (cell proteins or virus particles) to allow movement along the cytoskeleton. Thus, one hypothesis is that the interaction of the Sirevirus Gags with LC8 or LC6 may allow for movement of the Sirevirus virus-like particles 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.