The beet yellows closterovirus p65 homologue of HSP70 chaperones has ATPase activity associated with its conserved N-terminal domain but does not interact with unfolded protein chains

Abstract

The positive-strand RNA genome of beet yellows closterovirus (BYV) encodes a 65 kDa protein (p65) related to the HSP70 family of cell chaperones. The full-sized BYV p65, and N- and C-terminal fragments, with (His)6 tails, were overexpressed in bacteria and purified by metal-chelate chromatography. Using a polyclonal antiserum raised against the C-terminal fragment of p65, evidence was obtained for expression of the viral protein in planta. Purified recombinant p65 and its N-terminal 40 kDa fragment exhibited Mg2+-dependent ATPase activity in vitro. However, unlike its cellular HSP70 homologues, p65 was unable to bind to denatured protein and its ATPase activity was not stimulated by synthetic peptides which are known to stimulate HSP70 ATPases. Hence, the BYV p65, although being a chaperone-type ATPase, may have a distinct substrate specificity and function in BYV-infected cells.