FIGURE 1.

Schematic of a proposed accumulation and movement pathway for TMV within cells. To simplify the model we do not address the possibility that the MP or any other viral protein moves within the cell, with or without viral RNA, independently of the virus replication complex. Also, we do not address the possibility that host proteins involved in virus accumulation and movement traffic independently from the virus complexes to support these activities. TMV capsid enters through an opening within the cell wall (CW) and plasma membrane (PM) or through pinocytosis after wounding (a). TMV RNA is released from the capsid at the site of viral RNA (vRNA) granule formation (b). The granules are associated with the endoplasmic reticulum (ER), which may serve as the replication site on transport of the vRNA to cortical vertices or perinuclear regions of the ER. Transport to these locations requires microfilaments (MF) (c). Other membranes such as the vacuolar (V) membrane may serve as a scaffold for virus replication, but this requires further analysis. A virus replication complex (VRC) is formed in the cortical vertices or perinuclear region of the ER (d). VRCs contain vRNA, movement protein (MP), replication proteins and host proteins. TOM1, a membrane protein, interacts with replication proteins and serves as an anchor between the replication proteins and a host membrane, which may be ER (TOM1?), vacuole (TOM1) or another membrane (e). For TMV intercellular movement, VRCs move from sites of replication to plasmodesmata (PD). Elongation factor 1A (EF-1A) interacts with vRNA, replication proteins, MFs and microtubules (MTs) and influences TMV movement. It is unclear if this influence is on sustained movement associated with clearance of virus components within the cell, or with initial movement: we have placed it with initial movement and with the MF (f). An interaction between two host proteins, a class II KNOTTED 1-like protein (NTH201) and a DnaJ-like protein (MPIP1), and the TMV MP also may aid transport of virus to the PD (g), although again it is unclear if this interaction aids initial or sustained movement. Movement of the VRC to the PD requires membrane, and may be influenced by actomyosin (MF and myosin) and MT (h). The influence of the MT end-binding protein (EB1a) on virus movement is placed during transport to the PD (h). MP microfilament severing activity at the PD is proposed to eliminate F-actin-like structures at the PD to increase the PD size exclusion limit SEL (i). In addition, interaction between the MP and the ankyrin repeat containing protein (ANK) is correlated with an increase of the PD SEL through a decrease in callose at the PD neck (j). vRNA transports through PD within a VRC or simply with vRNA and MP, the latter being phosphorylated (MP^P) in the PD to release the vRNA for translation in the next cell (k; Karpova et al., 1997; Lee et al., 2005). After vRNA transfer to the neighboring cell, VRC remnants associate with endosomes (E; possibly pre-vacuolar vesicles) for transport to vacuoles, potentially through interaction of the vesicle fusing protein, synaptotagmin, with the MP (l). Transport is proposed to involve the actomyosin network. Likely prior to this transport, CELL-DIVISION CYCLE protein48 (CDC48) extracts the MP from the ER-associated VRC for attachment to the MT and later degradation (m). N, nucleus.