Plant cell and viral helicases: Essential enzymes for nucleic acid transactions

Helicases are ubiquitous enzymes that catalyze the unwinding of energetically stable duplex DNA (DNA helicases) or duplex RNA secondary structures (RNA helicases) and thus play essential role in all aspects of nucleic acid metabolism. All helicases share the common property of being able to use the energy derived from NTP hydrolysis (usually ATP) to break the hydrogen bonds that hold both the two strands together. Mechanistically, there are two classes of helicases: those that can translocate 3'- to 5'-, or 5'- to 3'- directions with respect to the strand on which they initially bind. DNA helicases are essential for key biological processes such as the DNA replication, repair, recombination, and transcription. Similarly, RNA helicases represent a large family of proteins that are involved in modulation of RNA structure and thereby influencing RNA synthesis, splicing, replication, translation initiation, editing, rRNA processing, ribosome assembly, nuclear mRNA export, mRNA stabilization, and degradation, which may influence important biological processes such as embryogenesis, cell growth, cell division, and signal transduction. Helicases are mainly present in nuclei, mitochondria, and chloroplasts of a green plant cell, where the DNA and RNA metabolism takes place. Plane virus-encoded RNA helicases have also been found, but DNA helicases have not yet been reported from plant viruses. The presence of conserved sequences (helicase motifs) among plant RNA helicases, plant virus-encoded RNA helicases, and the helicases (or putative helicases or DEAD-box proteins) from other eukaryotic and prokaryotic systems reveals a close relationship between them and suggests that these proteins might have been derived from a common ancestor. A variety of different helicases have been identified recently from plant cells and viruses, but their role has not been well investigated. In this article all the information from discovery to the current state of knowledge of plant DNA and RNA helicases and plant virus-encoded helicases, as well as their possible functions in nucleic acid transactions, is reviewed. In addition, various characteristics of helicases with respect to their ATPase activity, interaction with DNA or RNA, polarity of translocation, processivity, thermodynamic efficiency and rate of unwinding, analogy to motor proteins, oligomeric nature, and crystal structures have also been covered in detail in this review.