Increased accuracy of ligand sensing by receptor internalization

Many types of cells can sense external ligand concentrations with cell-surface receptors at extremely high accuracy. Interestingly, ligand-bound receptors are often internalized, a process also known as receptor-mediated endocytosis. While internalization is involved in a vast number of important functions for the life of a cell, it was recently also suggested to increase the accuracy of sensing ligand as the overcounting of the same ligand molecules is reduced. Here we show, by extending simple ligand-receptor models to out-of-equilibrium thermodynamics, that internalization increases the accuracy with which cells can measure ligand concentrations in the external environment. Comparison with experimental rates of real receptors demonstrates that our model has indeed biological significance.Comment: 9 pages, 4 figures, accepted for publication in Physical Review

Phospholipase C in Dictyostelium discoideum. Cyclic AMP surface receptor and G-protein-regulated activity in vitro.

The cellular slime mould Dictyostelium discoideum shows several responses after stimulation with the chemoattractant cAMP, including a transient rise in cyclic AMP (cAMP), cGMP and Ins(1,4,5)P3. In this paper the regulation of phospholipase C in vitro is described. Under our experimental conditions commercial PtdIns(4,5)P2 cannot be used to analyse phospholipase C activity in Dictyostelium lysates, because it is hydrolysed mainly to glycerophosphoinositol instead of Ins(1,4,5)P3. Enzyme activity was determined with endogenous unlabelled PtdInsP2 as a substrate. The product was measured by isotope-dilution assay and identified as authentic Ins(1,4,5)P3. Since phospholipase C is strictly Ca(2+)-dependent, with an optimal concentration range of 1-100 microM, cell lysates were prepared in EGTA and the enzyme reaction was started by adding 10 microM free Ca2+. Phospholipase C activity increased 2-fold during Dictyostelium development up to 8 h of starvation, after which the activity declined to less than 10% of the vegetative level. Enzyme activity in vitro increased up to 2-fold after stimulation of cells with the agonist cAMP in vivo. Addition of 10 microM guanosine 5'-[gamma-thio]triphosphate during lysis activated the enzyme to the same extent, and this effect was antagonized by guanosine 5'-[beta-thio]diphosphate. These results strongly suggest that surface cAMP receptors and G-proteins regulate phospholipase C during Dictyostelium development

G-proteins and the inositol cycle in Dictyostelium discoideum

The inositol cycle in Dictyostelium disocideum was studied both in vitro and in vivo. The results are compared to the inositol cycle as it is known from higher eukaryotes. Although there is a strong resemblance the cycles are different at some essential points. In comparison to higher eukaryotes, in the cycle in D. discoideum the inositol 1,4,5-trisphosphate [Ins(1,4,5) P3] kinase appears to be absent and there are additional phosphatases which hydrolyse Ins(1,4,5)P3 via inositol 4,5-bisphosphate [Ins(4,5)P2] to inositol 4-phosphate (Ins4P). The function of the receptor-stimulated inositol cycle was elucidated using mutants from the fgd A complementation group, which are defective in the G-protein alpha-subunit, responsible for the activation of phosphoinositidase C. These mutants show defects in both chemotaxis and differentiation, suggesting that the stimulation of phosphoinositidase C is the major sensory transduction pathway in D. discoideum.</p