Retrotransposon Target Site Selection by Imitation of a Cellular Protein

Abstract

Mobile elements rely on cellular processes to replicate, and therefore, mobile element proteins frequently interact with a variety of cellular factors. The integrase (IN) encoded by the retrotransposon Ty5 interacts with the heterochromatin protein Sir4, and this interaction determines Ty5's preference to integrate into heterochromatin. We explored the hypothesis that Ty5's targeting mechanism arose by mimicking an interaction between Sir4 and another cellular protein(s). Mutational analyses defined the requirements for the IN-Sir4 interaction, providing criteria to screen for cellular analogues. Esc1, a protein associated with the inner nuclear membrane, interacted with the same domain of Sir4 as IN, and 75% of mutations that disrupted IN-Sir4 interactions also abrogated Esc1-Sir4 interactions. A small motif critical for recognizing Sir4 was identified in Esc1. The functional equivalency of this motif and the Sir4-interacting domain of IN was demonstrated by swapping these motifs and showing that the chimeric IN and Esc1 proteins effectively target integration and partition DNA, respectively. We conclude that Ty5 targets integration by imitating the Esc1-Sir4 interaction and suggest molecular mimicry as a general mechanism that enables mobile elements to interface with cellular processes.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank P. James for the yeast two-hybrid library, M. Gartenberg for reagents for measuring mitotic stability, and R. Sternglanz for Esc1 plasmids and yeast strains for the heterochromatin nucleation assays. We are grateful to X. Gao for help with statistical analyses.

This work was supported by National Institutes of Health grant GM061657 to D.F.V.