MOLECULAR HETEROGENEITY OF THE SUBCLASSES OF ISLET-ACTIVATING PROTEIN (PERTUSSIS TOXIN)-SENSITIVE GTP-BINDING PROTEINS IN PORCINE THYROID-TISSUE

From porcine thyroid cell membranes, we purified five GTP-binding proteins (G-proteins); Nos. 1 to 3 have 41-kDa α-subunits, and Nos. 4 and 5 have 40-kDa α-subunits. They were chromatographically (Mono Q) separable and served as specific substrates for islet-activating protein (pertussis toxin). G-proteins 1 and 2 were indistinguishable from porcine brain Gi 1 with respect to three criteria, i.e., mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), pI of the ADP-ribosylated α-subunit, and immunoreactivity. G-protein 3 was identified as Gi 3 by immunoreactivity. The SDS-PAGE and isoelectric focusing (IEF) analyses identified G-proteins 4 and 5 as being chromatographically heterogeneous subtypes of Gi 2 in comparison with a pure porcine brain preparation. The IEF analysis also disclosed that each of the gi 1, Gi 2, and Gi 3 subspecies isolated in the present study has a minor component characterized by a slightly lower pI of its α-subunit. We conclude that porcine thyroid tissue contains at least Gi 1, Gi 2, and Gi 3, and that each is made up of heterogeneous populations. © 1990.