<P> In mammalian cells, the much - studied β - adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin - mediated uncoupling of G - protein - mediated signaling . Therefore, it seems likely that some mechanisms previously believed related purely to receptor desensitisation are actually examples of receptors switching their signaling pathway, rather than simply being switched off . </P> <P> In kidney cells, the bradykinin receptor B2 has been shown to interact directly with a protein tyrosine phosphatase . The presence of a tyrosine - phosphorylated ITIM (immunoreceptor tyrosine - based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin . </P> <P> Although it is a relatively immature area of research, it appears that heterotrimeric G - proteins may also take part in non-GPCR signaling . There is evidence for roles as signal transducers in nearly all other types of receptor - mediated signaling, including integrins, receptor tyrosine kinases (RTKs), cytokine receptors (JAK / STATs), as well as modulation of various other "accessory" proteins such as GEFs, guanine - nucleotide dissociation inhibitors (GDIs) and protein phosphatases . There may even be specific proteins of these classes whose primary function is as part of GPCR - independent pathways, termed activators of G - protein signalling (AGS). Both the ubiquity of these interactions and the importance of Gα vs. Gβγ subunits to these processes are still unclear . </P> <P> There are two principal signal transduction pathways involving the G protein - linked receptors: the cAMP signal pathway and the phosphatidylinositol signal pathway . </P>

Which of the following are the three subclasses of membrane receptors