A surprising effect of extraction conditions on the mobility of some plant virus coat proteins in SDS-PAGE.

The coat protein from purified particles of pea seedborne mosaic potyvirus (PSbMV) moves in SDS-PAGE with an apparent molecular weight (M(r)) of 36 kDa. However, extracts of PSbMV infected plants prepared with SDS or urea contain PSbMV immunoreactive proteins with apparent M(r) 39 kDa as well as 36 kDa. The low mobility form may be generated from the apparent M(r) 36 kDa form by incubating purified PSbMV particles with healthy plant sap in the presence of denaturing agents. A similar effect is observed with bean yellow mosaic potyvirus, but not with three viruses outside the potyvirus group. Experiments suggest that a soluble plant enzyme is responsible for the conversion, which apparently takes place only in vitro under denaturing conditions. This phenomenon may lead to erroneous conclusions about the M(r) of some viral coat proteins. However, the conversion can be prevented by heat treatment of the plant tissue prior to extraction.