G proteins detected by ADP-ribosylation

Under special conditions other proteins can be labeled with CTX. Notable among these are bands at Mr ca. 39000 in adipocyte membranes, which are observed only when the ADP-ribosylation is performed in the absence of GTP. These bands, la- 

Like CTX, PTX also ADP-ribosylates T [160]. But, the ADP-ribosylated amino acid is Cys347, four amino acids from the carboxyterminus of T [161]. 

Conditions for labeling with PTX differ markedly from those with CTX [162]. The abundance of PTX substrates in membranes is much greater than that of CTX substrates. Autoradiograms of labeling with PTX reveal fewer bands than those of with CTX and show ADP-ribosylation exclusively at Mt values that range from a maximum of 41000 to a minimum of around 39000. Under most circumstances the band at 41000 (which is quantitatively minor compared to the others) is not observed, leaving only a cluster of bands - mostly fused - with Mt from 41000 to 39000 (Fig. 6). 

It is currently not clear as to how many PTX substrates there are and, of these, how many may co-exist in any given tissue. Brain has been reported as containing three PTX substrates of Mr 39000, 40000 and 41000 [73]. In heart and fat two PTX substrates were reported [163-165], Human erythrocytes appear to have one PTX substrate of A/r 40000 [29] which can be fractionated into two [151]. Prolonged exposures of autoradiograms of human erythrocyte, bovine brain, human neutrophil and rat liver PTX substrates reveal still a third substrate of apparent Mt = 43 000 [166,167]. As illustrated in Fig. 7, even more, up to five distinct proteins, can be visualized if membrane proteins are fractionated by SDS-PAGE in the presence of an urea gradient [151]. The functional difference between these PTX-labeled bands is not known at present. 

These PTX-labeling studies raise an obvious question which PTX substrate mediates which coupling event  

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