The positively charged structural virus protein (VP1) of foot-and-mouth disease virus (type O1) contains a highly basic part which may be involved in early virus-cell interaction.

Polypeptides of 'trypsin-resistant' (TR) variants of foot-and-mouth disease virus type O1 (BFS 1860) were analysed by electrofocusing and two-dimensional gel electrophoresis. In contrast to parent O1 virus, trypsin treatment of these variants did not reduce their infectivity and their ability to attach to susceptible cells, although VP1 was cleaved as in the parent virus. In OTR1, one of the cloned isolates, an additional polypeptide (VPA) with a mol. wt. approx. 31 X 10(3) (31K), was found which resembled VP1 (28K) in being positively charged and cleaved by trypsinization of the virus into a neutral 18K polypeptide (P18) and a strongly basic fragment (pI greater than 10) with a mol. wt. of approx. 6K (P6). These findings substantiate the hypothesis that VPA is an elongated VP1. While P18 fragments of both trypsin-treated parent virus and OTR progeny viruses focused at identical (neutral) pH, P6 fragments of trypsinized OTR variants (including OTR1) were even more positively charged than P6 of parent virus. This difference in charge of the P6 polypeptide may be responsible for the retained cell attachment ability of trypsinized OTR viruses. The data are discussed with respect to the known amino acid sequence of VP1 of the closely related O1 Kaufbeuren.