Geminivirus protein structure and function

Abstract

Geminiviruses are a family of plant viruses that cause economically important plant diseases worldwide. These viruses have circular single-stranded DNA genomes and four to eight genes that are expressed from both strands of the double-stranded DNA replicative intermediate. The transcription of these genes occurs under the control of two bidirectional promoters and one monodirectional promoter. The viral proteins function to facilitate virus replication, virus movement, the assembly of virus-specific nucleoprotein particles, vector transmission and to counteract plant host defence responses. Recent research findings have provided new insights into the structure and function of these proteins and have identified numerous host interacting partners. Most of the viral proteins have been shown to be multifunctional, participating in multiple events during the infection cycle and have, indeed, evolved coordinated interactions with host proteins to ensure a successful infection. Here, an up-to-date review of viral protein structure and function is presented, and some areas requiring further research are identified.

Figure 1

Genome organization of representative members of the four geminivirus genera: Begomovirus, Mastrevirus, Curtovirus and Topocuvirus. Except for some begomoviruses, which have two component genomes, all geminiviruses have a single genome component. The bipartite begomovirus genome of a representative member, East African cassava mosaic Cameroon virus (EACMCV) (a) is composed of two DNA molecules, designated DNA‐A and DNA‐B, whereas Tomato yellow leaf curl virus (TYLCV) (b) is a single component genome begomovirus. Maize streak virus (MSV) (c) is a type member of mastreviruses, whereas Beet curly top virus (BCTV) (d) is the type member of curtoviruses and, so far, Tomato pseudo‐curly top virus (TPCTV) is the only member of the genus Topocuvirus (e). Open reading frames (ORFs) are represented by curved arrows and are named according to the DNA component and the DNA strand on which they are found (viral sense, V or complementary sense, C). Gene names are provided in the text. CP, coat protein; MP, movement protein; REn, replication enhancer protein; Rep, replication‐associated protein; TrAP, transcriptional activator protein.

Figure 2

The replication‐associated protein (Rep) and replication‐associated protein A (RepA) proteins. (a) The functional limits of the Rep domains for DNA cleavage/ligation are shown. DNA binding and protein interaction regions (characterized for begomoviruses and mastreviruses) are indicated. The DNA binding and cleavage/ligation domains contain the conserved motifs I (DNA binding), II (metal binding) and III (DNA cleavage and ligation) and the Geminivirus Rep Sequence (GRS). The N‐terminal half of Rep also contains the oligomerization domain, and binding sites for the replication enhancer protein (REn), retinoblastoma‐related protein (RBR), proliferating cell nuclear antigen (PCNA), GRIK, small ubiquitin‐related modifier (SUMO)‐conjugating enzyme (SUMO) and ATP. (b) The RepA C‐terminus contains the binding sites for RBR and geminivirus RepA‐binding (GRAB) transcription factors, as well as a promoter transactivation domain (TRA). It is important to note that, despite the sequence identity between Rep and RepA cleave/ligate and DNA binding domains, these domains have not yet been determined experimentally in RepA.

Figure 3

The transcriptional activator protein (TrAP) and C2 proteins. The N‐terminal half of the geminivirus TrAP or C2 contains a nuclear localization signal (NLS) within a basic region and a central region contains a zinc finger‐like motif and a domain that counters programmed cell death (PCD). The C‐terminal end contains an acidic region required for transcriptional activation. The acidic domain is missing in curtovirus C2 proteins.

Figure 4

The replication enhancer protein (REn). Several REn functional regions were elucidated on the basis of Tomato golden mosaic virus (TGMV), Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl Sardinia virus (TYLCSV). Two retinoblastoma‐related protein (RBR) interacting domains are located in both the N‐ and C‐terminal ends. Three hydrophobic clusters (HD), an oligomerization, Rep as well as proliferating cell nuclear antigen (PCNA) and SINAC1 (Solanum lycopersicum NAC1) binding domains have been identified in REn.

Figure 5

The coat protein (CP) contains three nuclear localization signal (NLS) motifs, the N‐terminal domain overlapping with the DNA binding domain. The central NLS is in the same region as the nuclear export signal (NES) and the cell wall targeting motif (CW).

Figure 6

The nuclear shuttle protein (NSP). The NSP N‐terminal half contains two nuclear localization signals (NLS‐A and NLS‐B), DNA binding and sequences required for the induction of the hypersensitive response, whereas the NSP C‐terminus contains the nuclear export signal (NES), movement protein (MP) binding site and the Arabidopsis nuclear shuttle protein interactor (AtNSI).

Figure 7

The movement protein (MP). The BC1 encoded MP contains a pilot domain required for localization to the cell periphery, a central anchor domain that also targets the protein to the cell periphery and a C‐terminal domain that facilitates oligomerization. The nuclear shuttle protein (NSP) binding site is in the centre of MP.