Native CD109 contains an intact thioester.

Boiling of radiolabeled KG1a-derived mAb 8A3–CD109 immune complexes results in the appearance of the typical 150- and 170-kd CD109 bands (arrows, lane 1) and of an additional 20-kd band (not shown). If, however, the immune complexes are treated with 400 mM methylamine before boiling, formation of the 150-kd band is greatly inhibited (lane 2). Thus, the 150-kd CD109 band arises by the thioester-mediated autolytic cleavage of the 170-kd form, which is abrogated if the thioester is first disrupted with the small nucleophile methylamine. Positions of molecular weight markers are shown on the right.