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The Consensus N-Myristoylation Motif of a Geminivirus AC4 Protein Is Required for Membrane Binding and Pathogenicity

April 2007 , Volume 20 , Number  4
Pages  380 - 391

Vincent N. Fondong , 1 R. V. Chowda Reddy , 1 Cheng Lu , 2 Bertrand Hankoua , 1 Christian Felton , 1 Kirk Czymmek , 2 and Fidelis Achenjang 3

1Department of Biological Sciences, Delaware State University, 1200 North DuPont Highway, Dover, DE 19901, U.S.A.; 2Delaware Biotechnology Institute, Delaware Technology Park, 15 Innovation Way, Newark DE 19711, U.S.A.; 3Union College, 310 College Street, Barbourville, KY 40906, U.S.A.

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Accepted 13 November 2006.

Some geminiviruses encode a small protein, AC4, whose role in pathogenesis has only recently attracted attention. A few studies have shown that this protein is involved in pathogenesis and suppresses RNA silencing. Here, using Nicotiana benthamiana, we show that East African cassava mosaic Cameroon virus (EACMCV) AC4 is a pathogenicity determinant and that it suppresses the systemic phase of RNA silencing. Furthermore, confocal imaging analyses show that it binds preferentially to the plasma membrane as well as to cytosolic membranes including the perinucleus but is excluded from the nucleus. A computational examination of the AC4 protein encoded by the EACMCV, a bipartite geminivirus, shows that it encodes a consensus N-myristoylation motif and is likely posttranslationally myristoylated and palmitoylated. Replacement of Gly-2 and Cys-3 (sites of posttranslational attachment of myristic and palmatic acids, respectively) with alanine affected AC4 membrane binding and pathogenesis. Furthermore, replacement of Ile-5, a nonessential myristoylation residue, with alanine did not affect AC4 function. Together, these data indicate that EACMCV AC4 is likely dually acylated at Gly-2 and Cys-3 and that these modifications are intrinsic signals for membrane targeting and pathogenesis. This is the first report of a membrane protein to be involved in pathogenesis and RNA silencing suppression.

Additional keywords: palmitoylation, RNA silencing suppression, subcellular localization.

© 2007 The American Phytopathological Society