We were surprised to find that pAtPSK4-myc was not cleaved in roots of intact transgenic seedlings (Figure 1a , wt, 0 time). We suspected that failure to detect processing may have been due to seedling growth or culture conditions. We had chosen to study AtSBT1.1 in the first place due to observations that we had made about shoot regeneration from root explants (Lall et al., 2004) . Therefore, we subjected root explants from 35S:ppAtPSK4-myc seedlings to tissue culture conditions for regenerating shoots. This involves pre-incubating root segments on an auxin-rich callus induction medium (CIM), and then transferring them after 4 days to a cytokinin-rich shoot induction medium (SIM). Under these conditions, a band appeared at approximately 7 kDa in Western blots (Figure 1a , wt, lanes 1d and 4d CIM or SIM), representing the myc-tagged cleaved peptide, AtPSK4-myc. The size of the processed protein was consistent with a cut at or near the cleavage site as determined by the in vitro experiments described below. Thus, cleavage of pAtPSK4myc appears to be induced by some aspect of the culturing process.