**6.** *Citrus tristeza virus* **encapsidation**

Differently of most elongated viruses, CTV particles are encapsidated by two different capsid proteins that coat the opposite ends of the virion [24]. About the 97% of the CTV genome is coated by CP, while the remainder 3% is coated by CPm resulting in viral particles with the emblematic tail of the members of the *Closteroviridae* family [11, 24]. The coordinate action of HSP70h and p61, in addition to the CP and CPm coat proteins, are required for proper assembly of CTV particles [13]. The previously described CTV 5´NTR conserved SL structures contains the origin of assembly of CPm, overlapping the sequences that function as a cis-acting element required for gRNA synthesis [24]. During CTV assembly, CPm begins coating the gRNA at the 5´ NTR to about nt 630. However, in the absence of HSP70h or p61, CPm may coat larger segments or even the complete gRNA. Probably, HSP70h or p61 bind to the transition zone between CP and CPm (around 630 nt) and restrict CPm to the virion tail [24]. The protein homologous to HSP70h and p61 in BYV are coordinately assembled with CPm in the virion structure and remain attached to the viral particles [58]. Although the assembly of HSP70h and p61 is not directly confirmed for CTV, RNA transcripts lacking one or both of these ORFs were unable to produce infective CTV virions [24]. Several strains with divergent genotypes could coexist in the same citrus cell. Analysis of the encapsidation of heterologous CPms in absence of HSP70h and p61 indicated reduction or lack of CPm assembly. Nevertheless, the presence of HSP70h and p61 restored CPm assembly to wild-type levels. This indicated that the HSP70h and p61 could play an important role facilitating heterologous CPm assembly in mixed infections [59].

The region coated by CPm also overlaps the LMT2 5´ sgRNA (650 nts). Actually, LMT2 production is correlated to virion assembly since mutations in the 5´ NTR that abolish encapsidation also eliminate accumulation of LMT2. Although this represents the first evidence of a viral RNA processed by the assembly mechanism, the exact function of LMT2 in CTV assembly is unknown at the moment [52].
