Regulation of adenylyl cyclases by a region outside the minimally functional cytoplasmic domains

Effects of alcohols on recombinant adenylyl cyclase type 7 expressed in bacteria

BACKGROUND

Our previous studies showed that ethanol enhanced the activity of adenylyl cyclase (AC) in an isoform-specific manner and that alcohol cutoff point of AC was isoform specific. Recently, we showed that 2,3-butanediol inhibited AC type 7 (AC7) activity in a stereoisomer-specific manner and that this inhibition was also AC isoform specific. These observations strongly suggest that a major target of alcohol action on cAMP signaling is AC. We hypothesized that alcohols exhibit their effect on AC activity by direct interaction with AC proteins. However, experimental systems employed in past studies such as intact cells and membrane preparations are too complex and do not allow us to unequivocally test this hypothesis. In attempt to bypass, these complications of the membrane-bound AC, we decided to study the effect of alcohols on AC recombinant proteins expressed in bacteria.

METHODS

A recombinant AC, designated as AC7sol, consisting of the C(1a) and C(2) domains of the human AC7 was designed and expressed in bacteria. The activity of AC7sol was examined using lysate prepared from bacteria expressing AC7sol.

RESULTS

The activity of AC7sol was stimulated by manganese or by the α subunit of G protein that stimulates AC (G(sα) ). Forskolin by itself did not stimulate the activity of AC7sol. However, in the presence of activated G(sα) , forskolin stimulated the activity of AC7sol. A series of n-alkanols including ethanol enhanced the manganese-stimulated activity of AC7sol. The alcohol cutoff point of AC7sol was pentanol. Ethanol and butanol increased V(max) and K(M) values of AC7sol.

CONCLUSIONS

These results are consistent with our hypothesis and suggest that the enhancing effect of alcohols on AC activity is because of the increase in turnover number of AC. The current study demonstrates for the first time that the effect of alcohols requires only the C(1a) and C(2) domains of AC and no other domains of AC as well as no other mammalian proteins.

Identification of ethanol responsive domains of adenylyl cyclase

BACKGROUND

The activity of adenylyl cyclase (AC) is enhanced by pharmacologically relevant concentrations of ethanol. The enhancing effect of ethanol on AC activity is AC isoform-specific. Therefore, we hypothesized that within a cyclic AMP-generating system, AC is the target of ethanol's action and that ethanol-sensitive AC molecules contain structural elements modulated by ethanol. The structural elements are designated as "ethanol responsive domains."

METHODS

By using a series of chimeric mutants, we searched regions of the AC molecule that are important for the ethanol effect. These chimeric mutants were derived from 3 isoforms of AC: AC7 (type 7), the most ethanol responsive isoform; AC3 (type 3), an isoform that is far less responsive to ethanol; and AC2 (type 2), an isoform that is homologous to AC7 but less responsive to ethanol.

RESULTS

We identified 2 discrete regions of the AC molecule that are important for the enhancement of AC activity by ethanol. The first is the N-terminal 28-amino-acid (aa) region of the C(1a) domain. The second is the C-terminal region ( approximately 140 aa) of the AC molecule. Sequence differences in the N-terminal tail, 2 putative transmembrane domains, and the C(1b) domain are not important for ethanol's effect.

CONCLUSIONS

The current study with mammalian ACs provides a new class of alcohol-responsive protein and possibly a new mechanism of alcohol action on cellular function. The identification of ethanol responsive domains will facilitate the elucidation of the mechanisms by which ethanol enhances the activity of AC.

Functional analysis of the interface regions involved in interactions between the central cytoplasmic loop and the C-terminal tail of adenylyl cyclase

The mammalian adenylyl cyclase is a membrane-bound enzyme that is predicted to have 12 trans-membrane spans. Between membrane spans 6 and 7 there is a large cytoplasmic loop, which, along with the C-terminal tail, makes up the catalytic site of the enzyme. Crystal structures of these soluble cytoplasmic domains have identified the regions that are involved in interactions with each other. The functional consequences of these interactions in the full-length membrane-embedded enzymes have not been established. In this study, we analyzed the role of various interaction regions within the central cytoplasmic loop (C1) and the C-terminal tail (C2) on basal, Galphas-, forskolin-, and Mn(2+)-stimulated activities of adenylyl cyclases 2 and 6 (AC2 and AC6). We tested synthetic peptides encoding the different interface surfaces of both the C1 and C2 domain on different activities of membrane-bound AC2 and AC6 expressed in insect cells. We found the C1-alpha2-beta2-beta3 and C2-beta2'-beta3' regions to be involved in stimulation by Galphas and forskolin but not in the basal or Mn(2+)-stimulated activities. Both the C1-beta4-beta5-alpha4 region and the C2-alpha3'-beta4' region play a role in the Galphas- and forskolin-stimulated activities as well as in basal activity, because the peptides encoding these regions inhibit basal activity by 30%. In contrast, the C2-alpha2' region peptide inhibits both basal and Mn(2+)-stimulated activity by >50%. These results suggest that the different stimulated activities may involve distinct interface interactions in the intact enzyme and, consequently, the distinct mechanisms by which Mn(2+) activates the enzyme as compared with Galphas and forskolin, leading to the possibility that the full-length adenylyl cyclase may have multiple catalytically competent configurations.

Protein kinase C inhibits type VI adenylyl cyclase by phosphorylating the regulatory N domain and two catalytic C1 and C2 domains

We previously showed that phosphorylation of Ser(10) of the N terminus domain of the type VI adenylyl cyclase (ACVI) partly mediated protein kinase C (PKC)-induced inhibition of ACVI. We now report that phosphorylation of the other two cytosolic domains (C1 and C2), which form the catalytic core complex of ACVI, also contributes to PKC-mediated inhibition. In vitro phosphorylation by PKC of the recombinant C1a and C2 domains, and of the synthetic peptides representing potential PKC phosphorylation sites, suggests that Ser(568) and Ser(674) of the C1 domain and Thr(931) of the C2 domain might act as substrates for PKC. We next created several full-length ACVI mutants in which one or more of the four likely PKC phosphorylation sites (Ser(10), Ser(568), Ser(674), and Thr(931)) were mutated to alanine. Simultaneous mutation of at least two of the three likely residues located in the N and C1 domains (Ser(10), Ser(568), and Ser(674)) was required to render ACVI variants completely insensitive to PKC treatment. In contrast, a single mutation of Thr(931) was sufficient to create a functional ACVI mutant that exhibited no detectable PKC-mediated inhibition, demonstrating the essentiality of Thr(931) to PKC-mediated regulation. Based on these results, we propose that the three cytosolic domains of ACVI might form a regulatory complex. Phosphorylation of this regulatory complex at different sites might induce a fine-tuning of the catalytic core complex and subsequently lead to alternation in the catalytic activity of ACVI.