Search

Recent crystallographic studies revealed that G-protein-coupled receptors (GPCRs) possess a putative cytoplasmic helical domain, termed helix 8, at the proximal region of the C-terminal tail. However, the significance of helix 8 in GPCR functions and signaling is not fully understood. Helix 8 mutants of leukotriene B4 receptor type 1 (BLT1) exhibit prolonged activation after ligand stimulation, suggesting some regulatory roles of helix 8 in GPCR signaling. Here, we report the inhibitory role of BLT1 helix 8 on ligand-dependent internalization using BLT1 and platelet-activating factor receptor as model GPCRs. Mutating die dileucine motif in helix 8 of BLT1 to alanines (BLT1 LLAA) enhanced LTB4-dependent internalization of BLT1, whereas wild-type (WT) BLT1 exhibited minimal internalization. Mutational studies revealed diat phosphorylation of 5 serine/threonine residues between amino acids 308 and 319 of BLT1 was responsible for enhanced ligand-dependent internalization of BLT1 LLAA. BLT1 LLAA showed enhanced basal and ligand-dependent phosphorylation compared to WT BLT1. Taken together, helix 8 of BLT1 inhibits receptor internalization by suppressing the excessive phosphorylation of the C-terminal tail. [ABSTRACT FROM AUTHOR]

In order to identify proguanylin-secreting cells, we have raised an antiserum against the synthetic fragment of human proguanylin (1–15) and have examined the proguanylin-positive cells in the human and rat gastrointestinal tract by immunohistochemical methods. Numerous proguanylin (1–15)-immunoreactive cells were found in the gastrointestinal tract. They were either pyramidal or spindle shaped in the stomach. Spindle-shaped cells, frequently possessing long slender processes, were located at the base of the pyloric epithelium and did not extend to the lumen. In the duodenum and jejunum, these cells were mostly pyramidal in shape and often had a slender process towards the lumen. The immunostaining was completely blocked by the human proguanylin (1–15) fragment. Paneth and goblet cells were negative against this antiserum. The number of serotonin-positive cells was much larger than that of proguanylin-positive cells in all the segments tested. The number of proguanylin-positive cells decreased from the jejunum to the ileum and very few cells were observed in the colon. In contrast to serotonin-positive cells, most somatostatin-positive cells were also positive for proguanylin. Thus, proguanylin (1–15) or its related protein appears to coexist with somatostatin in intestinal endocrine D cells which may be a source of circulating proguanylin. Proguanylin, like somatostatin, may also regulate intestinal function as a local regulator. [ABSTRACT FROM AUTHOR]