Tomato leaf curl Yunnan virus-encoded C4 induces cell division through enhancing stability of Cyclin D 1.1 via impairing NbSKη -mediated phosphorylation in Nicotiana benthamiana

Abstract

The whitefly-transmitted geminiviruses induce severe developmental abnormalities in plants. Geminivirus-encoded C4 protein functions as one of viral symptom determinants that could induce abnormal cell division. However, the molecular mechanism by which C4 contributes to cell division induction remains unclear. Here we report that tomato leaf curl Yunnan virus (TLCYnV) C4 interacts with a glycogen synthase kinase 3 (GSK3)/SHAGGY-like kinase, designed NbSKη, in Nicotiana benthamiana. Pro32, Asn34 and Thr35 of TLCYnV C4 are critical for its interaction with NbSKη and required for C4-induced typical symptoms. Interestingly, TLCYnV C4 directs NbSKη to the membrane and reduces the nuclear-accumulation of NbSKη. The relocalization of NbSKη impairs phosphorylation dependent degradation on its substrate-Cyclin D1.1 (NbCycD1;1), thereby increasing the accumulation level of NbCycD1;1 and inducing the cell division. Moreover, NbSKη-RNAi, 35S::NbCycD1;1 transgenic N. benthamiana plants have the similar phenotype as 35S::C4 transgenic N. benthamiana plants on callus-like tissue formation resulted from abnormal cell division induction. Thus, this study provides new insights into mechanism of how a viral protein hijacks NbSKη to induce abnormal cell division in plants.

Nucleocytoplasmic Shuttling of Geminivirus C4 Protein Mediated by Phosphorylation and Myristoylation Is Critical for Viral Pathogenicity

“…Our previous studies have demonstrated that C4 interacts with NbSKh (Mei et al, 2018). This raised a possibility that TLCYnV C4 is a substrate of this kinase.…”

The Ever-Expanding Role of C4/AC4 in Geminivirus Infection: Punching above Its Weight?

“…Geminivirus C4/AC4 protein appears to exhibit similar functions to human viral ''oncogenes''. In support of this, Mei et al (2018b) showed that TLCYnV C4 induces cell cycle progression by sequestering SKh kinase, an inhibitor of cyclin CycD1;1. This action results in the accumulation of CycD1;1, which can then form a complex with CDK in the nucleus, enabling transition of the cell from G1 to S phase, and this ensures viral genome replication (Mei et al, 2018b).…”

“…In support of this, Mei et al (2018b) showed that TLCYnV C4 induces cell cycle progression by sequestering SKh kinase, an inhibitor of cyclin CycD1;1. This action results in the accumulation of CycD1;1, which can then form a complex with CDK in the nucleus, enabling transition of the cell from G1 to S phase, and this ensures viral genome replication (Mei et al, 2018b). These findings suggest that the ability of C4/AC4 to induce hyperplasia and tumorigenic growth in infected plant tissues is at least partially due to C4-mediated sequestration of SKh from the nucleus.…”

“…Recent studies by Mei et al (2018aMei et al ( , 2018b have provided new insights into the role of C4/AC4 during geminivirus infection and disease development. This work of Mei et al (2018a) focused on an N-terminal myristoylation sequence motif that is highly conserved in the otherwise highly variable geminivirus C4/AC4 protein sequences.…”

“…N-myristoylation is the irreversible posttranslational attachment of the hydrophobic myristoyl group to glycine at the second position (Gly2) in the protein sequence, giving the protein a hydrophobic characteristic that facilitates binding to cellular membranes. Although N-myristoylation typically occurs co-translationally on newly synthesized polypeptides in the cytoplasm following enzymatic cleavage of the first methionine (Met1), Mei et al (2018a) found that Tomato leaf curl Yunnan virus (TLCYnV) C4 is N-myristoylated in the nucleus, and this myristoylation is dependent on the phosphorylation of the threonine 51 (Thr51) by SKh, a member of plant SHAGGY-like kinases (Mei et al, 2018a(Mei et al, , 2018b. Interestingly, after these modifications, the C4-SKh complex shuttles back to the cytoplasm by interacting with exportin-a.…”