|Gowda, Siddarame -|
|Dawson, William -|
Submitted to: Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2010
Publication Date: May 19, 2010
Repository URL: http://hdl.handle.net/10113/44231
Citation: Tatineni, S., Gowda, S., Dawson, W.O. 2010. Heterologous Minor Coat Proteins of Citrus Tristeza Virus Strains Affect Encapsidation, but the Coexpression of HSP70h and p61 Restores Encapsidation to Wild-Type Levels. Virology. Volume 402:262-270. Interpretive Summary: Citrus tristeza virus (CTV) is the most economically important virus of citrus because it destroyed citrus industries in several countries during the last century, thus changing the course of the citrus industry. The long flexuous virions of CTV are encapsidated with two capsid proteins: the minor coat protein (CPm) encapsidates the 5' ~630 nucleotides of the genomic RNA and the major coat protein (CP) encapsidates the remainder of the genomic RNA, resulting in the formation of ‘rattlesnake-like’ virions. In this study, we found that encapsidation by CPm is highly specific in the absence of other assembly related proteins, CP, HSP70h and p61. However, the specificity of CPm encapsidation was mitigated in the presence of HSP70h and p61 proteins, but not HSP70h or p61 alone. Thus, we found an additional function for the HSP70h and p61 proteins in the assembly of complex virions of CTV–facilitating the initiation of assembly by heterologous CPm.
Technical Abstract: The long flexuous bipolar virions of Citrus tristeza virus (CTV), a Closterovirus, are encapsidated with two capsid proteins at opposite ends: the minor coat protein (CPm) encapsidates the 5’ 630 nts of the genomic RNA and the major coat protein encapsidates the remainder of the genome. In this study, we found encapsidation of CTV CPm in the absence of other assembly-related proteins is highly specific in contrast to most plant viruses that allow virion assembly by a range of heterologous coat proteins. Heterologous CPm’s with 95-96% amino acid identity from related strains in CTV-CPm, a replicon with CPm as the only assembly-related ORF, either failed to initiate encapsidation or reduced encapsidation substantially. Substitution of subsets of amino acids revealed that the amino acids that differ between positions 121 and 180 of the VT strain, and 61 and 120 of the T3 strain were involved in specific encapsidation. We further mapped the specific encapsidation to a single amino acid: mutation of methionine165 to threonine (VT type) or serine105 to proline (T3 type) in CTV-CPm failed to form nucleocapsids. However, the heterologous CPm in combination with both HSP70h and p61 proteins, but not HSP70h or p61alone, encapsidated at wild-type levels, suggesting that specific encapsidation by CPm was mitigated by the combination of HSP70h and p61. Thus, in addition to the previously described functions of HSP70h and p61 of greatly enhanced virion formation and restriction of CPm encapsidation to the 5’ 630 nts of the genomic RNA, these proteins facilitate encapsidation by heterologous CPm’s.