To determine whether AtSBT1.1 is involved in the proteolytic processing of AtPSKs, we developed a constitutively expressed, C-terminal 4 x myc-tagged construct of the AtPSK4 precursor, 35S:ppAtPSK4-myc, and studied its processing in vivo. PreproAtPSK4 will be referred to as ppAtPSK4 and proAtPSK4 as pAtPSK4. AtPSK4 was chosen for study because it is the most abundantly expressed PSK precursor . The predicted size of pAtPSK4 with the myc tag is 12.8 kDa, but we observed a band at approximately 19 kDa on Western blots, larger than the predicted size ( Figure 1a , wt, 0 time). To demonstrate, nonetheless, that this band is pAtPSK4myc, we analyzed the partially purified myc-tagged protein and identified three peptides derived from the fusion protein by MS/MS analysis ( Figure S1 ). Thus, the larger apparent size of pAtPSK4-myc may be due to anomalous gel migration behavior or post-translational modification of part of the precursor protein. (a) Root segments from wild-type transgenic seedlings expressing the construct 35S:ppAtPSK4-myc were explanted and incubated on callus induction medium (CIM) for 1-4 days and then transferred onto shoot induction medium (SIM). Root segments from sbt1.1-1 and sbt1.1-2 mutants expressing 35S:ppAtPSK4-myc were also explanted and similarly incubated. Arrows indicate the predicted migration position for processed AtPSK4-myc. The lane marked NT is an extract from roots of non-transformed seedlings.