Electron microscopy of the large form of transcarboxylase with six attached subunits.

In this paper we show that the native form of transcarboxylase may be a species which has six rather than three subunits attached to the central subunit. We have designated this form as the 26 S enzyme. Electron micrographs support the view that the six subunits are attached in sets of three at the opposite faces of the central subunit, in contrast to the 18 S form in which all three subunits appear to be attached only at one face. In addition, evidence is presented that the dissociation of the 26 S to the 18 S form of transcarboxylase occurs with the loss of three subunits exclusively from one face of the central subunit. This result may indicate that the two faces of the central subunit differ structurally or there is negative cooperativity in the dissociation of subunits. The 26 S transcarboxylase, which was made by attachment of subunits to the 18 S enzyme or trypsin-treated 18 S enzyme was shown to have subunits on both faces of the central subunit. Treatment of the 26 S enzyme with carbodimide to cross-link the subunits to the central subunit and thus stabilize the structure resulted in improved electron micrographs. A model of the 26 S form of the enzyme is presented.