XCP1 processing in wild-type xylem is different from that in 35S-XCP1 plants, and the 36-kD form of XCP1 can be activated in vitro. A, Polypeptides detected with anti-XCP1 antibody at 36 and 45 kD are common to wild-type xylem and 35S-XCP1 plants. Immunoreactivity at 29 and 18 kD is unique to xylem. B, In vitro autocatalytic processing of the 36-kD form of XCP1 excised from an SDS-PAGE gel resulted in proteolytic activity at 23 kD, and the loss of protein at 36 kD. For the denatured control, degradation and activation of the 36-kD protein was not observed.