Molecular evolution of ACTIN RELATED PROTEIN 6, a component of SWR1 complex in Arabidopsis

To date, it has been assumed that the evolution of a protein complex is different from that of other proteins. However, there have been few evidences to support this assumption. To understand how protein complexes evolve, we analyzed the evolutionary constraints on ACTIN RELATED PROTEIN 6 (ARP6), a component of the SWR1 complex. Interspecies complementation experiments using transgenic plants that ectopically express transARP6s (ARP6s from other organisms) showed that the function of ARP6s is conserved in plants. In addition, a yeast two-hybrid analysis revealed that this functional conservation depends on its ability to bind with both PIE1 and AtSWC6. ARP6 consists of 4 domains similar to actin. Functional analysis of chimericARP6s (domain-swapped ARP6s between Arabidopsis and mouse) demonstrated that each domain of ARP6s imposes differential evolutionary constraints. Domains 1 and 3 of ARP6 were found to interact with SWC6 and PIE1, respectively, and domain 4 provides a nuclear localization signal. Moreover, domains 1 and 3 showed a slower evolution rate than domain 4, indicating that the interacting domains have higher evolutionary constraints than non-interacting domains do. These findings suggest that the components of this protein complex have evolved coordinately to preserve their interactions.