Protein blot analysis of 5–30% glycerol density gradient fractions. Blots were probed with antibody to either BT2 or SH2 as indicated. A peak of BT2 and SH2 protein is identified in fraction 5 in the nonheated (CE) extracts of Sh2/Bt2 and Sh2hs33/Bt2, a result consistent with the peak of AGP activity (Fig. 5A). In the heated extract (HT) of Sh2/Bt2, BT2 and SH2 antigen levels are abolished in fraction 5, which contains the peak of AGP activity, whereas significant levels of BT2 and SH2 remain in the heated Sh2hs33/Bt2 extract. This finding indicates the mutation in Sh2hs33 not only enhances the interaction with the BT2 subunit, but in turn, stabilizes the tetrameric nature of AGP.