Evidence to support a spectrum of active states for the glucagon receptor.

The ternary complex model suggests that G-protein-coupled receptors resonate between inactive (R) and active (R*) forms. Physiologically, R sites ordinarily predominate with a few R* sites giving rise to basal activity. Agonists recognize, stabilize and increase the R* population, thus altering intracellular activity. There is evidence to suggest the possibility of a spectrum of conformations between R and R*. Our aim is to study the consequences of putative GR (glucagon receptor)-activating mutations using glucagon and partial agonist des-His(1)-[Glu(9)]glucagon amide (glucagon-NH(2)). Alanine substitution in TM (transmembrane) helix 2 of Arg(173) or of His(177) detrimentally affected glucagon and glucagon-NH(2) response maxima. TM2 receptor mutant, Phe(181)-Ala, displayed reduced maximum cAMP accumulation in response to glucagon-NH(2). Thr(353)-Cys (TM6) and Glu(406)-Ala (TM7) receptors demonstrated constitutive activity and enhanced EC(50) values for glucagon-NH(2); Arg(346)-Ala (TM6) and Asn(404)-Ala (TM7) receptors were activated by sub-fmol glucagon concentrations, yet were not constitutively active and demonstrated wild-type receptor-like EC(50) values for glucagon-NH(2). Unlike Arg(346)-Ala receptors, Thr(353)-Cys, Asn(404)-Ala and Glu(406)-Ala receptors demonstrated improved EC(50) values for glucagon, whereas their maximal responses to and their affinity for glucagon were comparable with the wild-type receptor. In contrast, despite slightly reduced glucagon-NH(2) affinity, Arg(346)-Ala, Thr(353)-Cys, Asn(404)-Ala and Glu(406)-Ala receptors displayed glucagon-NH(2) response maxima that exceeded those seen for wild-type receptors. Interestingly, we observed biphasic glucagon-mediated signalling responses. Our results are consistent with the concept of different agonists promoting the formation of distinct active states from partially active R*(low) to fully active R*(high) forms.

Expression of the abnormal gravitropism phenotypes creep and ageotropum during development in pea.

This study established that the mutant creep and ageotropum phenotypes are expressed differently during development in pea (Pisum sativum L.). Etiolated ageotropum stems grew at a wide range of angles, whereas etiolated creep stems emerged vertically from the compost. However, when etiolated creep stems were subjected to additional gravitropic demands, such as growth to excessive height or reorientation, abnormality was detectable. When plants were handled to a greater extent, earlier loss of vertical growth resulted. In light-grown shoots, creep lost vertical orientation, whereas ageotropum grew more normally. Root systems of creep exhibited normal growth patterns, whereas ageotropum main and lateral roots grew at abnormal angles. Thus, the ageotropum mutation strongly affects gravitropism in roots and etiolated stems, whereas the creep mutation affects both light- and dark-grown stems, but is most apparent in older, taller plants.