The alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein forms high-molecular-mass aggregates, concomitant with iloprost-induced desensitization of human platelet adenylyl cyclase

Selective formation of Gsalpha-MHC I complexes after desensitization of human platelets with iloprost

Prolonged treatment of human platelets with the adenylate cyclase-stimulating prostacyclin analog iloprost leads to reduction in cAMP formation. Previous studies have demonstrated that this may be ascribed to modification of both receptor and Gsalpha function rather than of the catalytic component of adenylate cyclase [Mollner, S., Deppisch, H. & Pfeuffer, T. (1992) Eur. J. Biochem. 210, 539-544]. Iloprost-induced desensitization was accompanied by the formation of a Gsalpha-containing 90-kDa product in membranes treated with the bifunctional cross-linker 1,6-bismaleimidohexane. The cAMP-inducing prostanoid PGD2, which does not promote desensitization, did not cause formation of the 90-kDa species either. The long-term effect of the common G-protein activator [AlF4]- on human platelet adenylate cyclase was shown in many respects to be comparable with that of iloprost. However, [AlF4]- treatment also failed to induce the 90-kDa species, showing that different mechanisms of desensitization were operating. Treatment of the cross-linked 90-kDa complex with PNGase F demonstrated the glycoprotein nature of the Gsalpha-associated component. The 90-kDa cross-linked product was purified by consecutive immunoaffinity chromatography and preparative PAGE to apparent homogeneity. Analysis of the purified protein by MS suggested that, besides Gsalpha, the heavy chain of MHC I (HLA-A2) was part of the complex. This was confirmed by coprecipitation of Gsalpha by the monoclonal anti-(MHC I) antibody W6/32.

Adenylyl cyclase type II is stimulated by PKC via C-terminal phosphorylation

Adenylyl cyclases of the type II family differ from other subforms in that they are conditionally stimulated via alpha(s)/betagamma subunits and regulated by PKC mediated phosphorylation. AC II, stably expressed in HEK 239 cells, was incubated with the PKC activator tetradecanoylphorbol acetate (TPA). Using cells metabolically labeled with [32P]phosphate, TPA caused concerted stimulation of basal and forskolin activated adenylyl cyclase together with incorporation of [32P]phosphate into AC II protein. Enhanced phosphorylation was also indicated by a monoclonal anti-phosphothreonine antibody. Assignment of TPA-induced [32P]phosphate-incorporation to specific sites was achieved by a combination of chemical and immunochemical methods. Three out of five [32P]labeled peptides that were generated by fragmentation with N-chlorosuccinimide were also recognized by the monoclonal antibody BBC-4 [S. Mollner, T. Pfeuffer, Eur. J. Biochem. 171 (1988) 265-271] directed against an epitope 8 kDa from the extreme C-terminus. These findings suggested Ser-871 (consensus sequence ARSLK) and Thr-1057 (CTCR) as acceptor candidates of phorbolester induced phosphoryl transfer.

Localisation of an ATP-binding site on adenylyl cyclase type I after chemical and enzymatic fragmentation

Photolabeling of partially purified bovine brain adenylyl cyclase (AC I) with [gamma 32P]8-N3-ATP led to incorporation of 32P into the 115 kDa catalyst. Further treatment with N-chlorosuccinimide, which cleaves proteins at tryptophan residues, yielded a 14 kDa 32P-labeled fragment. The latter was immunoprecipitated by antibody BBC1, recognizing the extreme C-terminus of AC I, but not by antibody BBC2, recognizing a more remote epitope. Further fragmentation of photolabeled AC I by the proteases Glu-C and Asp-N yielded 32P-labeled peptides corresponding to 2.9 kDa and 5.6 kDa fragments, which were not recognized by any of these antibodies. This narrows the ATP binding site down to a 25 amino acid sequence containing a general motif G(X0-7)KG(X0-4)L/M(X5-7)S/T present in all eukaryotic adenylyl cyclases so far cloned, but also in a variety of bacterial adenylyl cyclases (Peterkofsky et al. (1993) Progr. Nucleic Acids Res. Mol. Biol. 44, 31-65).

Acylation of adenylyl cyclase catalyst is important for enzymic activity

Incubation of human thrombocytes in the presence of [3H]palmitic acid leads to incorporation of this fatty acid into the alpha subunit of Gs as described [Linder et al., Proc. Natl. Acad. Sci. USA 90 (1993) 3675-3679; Degtyarev et al., Biochemistry 32 (1993) 8057-8061] but also into the catalyst of adenylyl cyclase which has not been recognized before. Treatment of labeled membranes with hydroxylamine released the label from both components. Label incorporated into the catalyst could be identified as [3H]palmitate. At the same time chemical deacylation caused partial loss of adenylyl cyclase activity.