Contents  ›  Introduction  ›  Chapter I  ›  Chapter II  ›  Chapter III  ›  Chapter IV  ›  Chapter V

Chapter V: The catalytic and regulatory domain of adenylate cyclase

Escherichia coli adenylate cyclase is a monomeric protein of 848 amino acids and molecular weight 97,586.

The existence of a catalytic domain was foreseen by Wang, Clegg and Koshland, Jr. in an elegant article published in 1981 [Proc Natl Acad Sci U S A].  It was found 'intriguing' that a 67,000 dalton peptide had some activity, although it had less activity than an 80,000 dalton one.  Since then, many truncated adenylate cyclases have been characterized that retained some activity.  In the 1981 PNAS article, it was justly argued that an essential activator must be present to obtain full adenylate cyclase activity, and 'perhaps' it was the product of the crr gene that encodes Enzyme IIAGlc.

In 1983, Roy et al. proposed adenylate cyclase to be composed of two functional domains, an amino-terminal catalytic domain and a carboxy-terminal regulatory domain.  Such proposal was related to the observation that the carboxy-terminal domain was required for the glucose-mediated regulation of adenylate cyclase [PubMed].  The existence of two functional domains was further emphasized upon availability of the adenylate cyclase gene full sequence [Nucleic Acids Res].

The catalytic domain of Escherichia coli adenylate cyclase was further characterized by Holland, Leib and Gerlt as a tryptic fragment of molecular weight 30,000, with an amino- and a carboxy-terminus corresponding respectively to residue 82 and 341 of adenylate cyclase [J Biol Chem].  Surprisingly, the 30,000 dalton peptide had kinetic properties similar to those reported for full-length adenylate cyclase.  Presently, this unexpected finding may be justified by postulating full activity of adenylate cyclase is reached only upon activation by phosphorylated Enzyme IIAGlc.

In 1994, a genetic approach provided the characterization of a 48,000 dalton amino-terminal domain that synthesized in vivo 10 times more cAMP than wild type adenylate cyclase.  The increased cAMP synthesis by this 'over-active' domain was observed in strains lacking Enzyme IIAGlc [Mol Genet Genomics].  Consequently a molecular model of interaction between domains was proposed describing the regulatory carboxy-terminal domain as inhibitory to the activity of the catalytic domain.  Subsequently it was tempting to suggest that phosphorylated Enzyme IIAGlc activates adenylate cyclase by relieving inhibitory interactions between domains.

Unfortunately in vitro data are not presently available that will demonstrate a direct interaction between the regulatory domain of adenylate cyclase and phosphorylated Enzyme IIAGlc.

In 2005, Strozen, Langen and Howard considered unlikely that the carboxy-terminal domain of adenylate cyclase is (i) necessary for the glucose-mediated regulation of adenylate cylase and (ii) inhibitory to the catalytic domain [J Bacteriol].  However the experimental data supporting such conclusions were scarce and the two adenylate cyclase mutants analyzed (C-terminal truncated adenylate cyclase of presumably 507 and 631 amino acids) were not characterized, especially by cAMP measurements.  These mutants were reported to be less active than wild type adenylate cyclase.  Because the rescuing effect observed with these mutants could not be reproduced in wild type strains grown on glucose, the conclusion, the carboxy-terminal domain of adenylate cyclase is not regulatory, is not experimentally supported.

In 2006, the proposal that the carboxy-terminal domain is regulatory upon interaction with phosphorylated Enzyme IIAGlc was reiterated [J Biol Chem].

The simplistic view that adenylate cyclase is composed of two functional domains does not reflect the complexity of the molecular interactions occurring within its likely multiple structural domains.  It does, however, reflect most of the experimental data obtained so far in vivo.

Chapter VI