Structural and biochemical characterization of cauliflower mosaic virus reverse transcriptase.

Area of Science:

• Biochemistry
• Structural Biology
• Virology

Background:

• Reverse transcriptases (RTs) are crucial enzymes in retroviral and retroelement replication, possessing DNA polymerase and RNase H activities.
• Caulimoviridae, a family of plant viruses, utilize RTs for their dsDNA genome replication, distinguishing them from retroviruses by lacking integrase.
• While Ty3 RT is well-characterized, structural and functional data for caulimoviral RTs remain limited.

Purpose of the Study:

• To determine the first crystal structure of cauliflower mosaic virus (CaMV) RT.
• To elucidate the functional mechanism of CaMV RT, particularly its polymerase and RNase H activities.
• To compare the structure and function of CaMV RT with other related RTs.

Main Methods:

• X-ray crystallography was used to obtain the structure of CaMV RT in complex with an RNA/DNA hybrid.
• Enzyme activity assays were performed to assess RNA-dependent DNA polymerase, DNA-dependent DNA polymerase, and RNase H functions.
• Biochemical analyses were conducted to investigate the quaternary structure and functional requirements of CaMV RT.

Main Results:

• The crystal structure revealed that CaMV RT forms a monomeric complex with the RNA/DNA hybrid, unlike the dimeric Ty3 RT.
• Individual CaMV RT molecules demonstrated full DNA polymerase activity (both RNA-dependent and DNA-dependent).
• RNase H activity required the transient association of an additional CaMV RT molecule with the primary polymerase-competent enzyme.

Conclusions:

• Cauliflower mosaic virus RT exhibits a unique monomeric structure during polymerase activity.
• A distinct mechanism involving transient dimerization is necessary for the RNase H function of CaMV RT.
• These findings provide novel insights into the structure-function relationship of caulimoviral RTs and their evolutionary relationship to other RTs.