No abstract

Abstract. Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly waterpermeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 -AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca 2+ ] i and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca 2+ mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca 2+ signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/ cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca 2+ ] i . In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

Rab5 is a Ras-related GTP-binding protein that is post-translationally modified by prenylation. We report here that an N-terminal domain contained within the first 22 amino acids of Rab5 is critical for efficient geranylgeranylation of the protein's C-terminal cysteines. This domain is immediately upstream from the "phosphate binding loop" common to all GTP-binding proteins and contains a highly conserved sequence recognized among members of the Rab family, referred to here as the YXYLFK motif. A truncation mutant that lacks this domain (Rab523-215) fails to become prenylated. However, a chimeric peptide with the conserved motif replacing cognate Rab5 sequence (MAYDYLFKRab523-215) does become post-translationally modified, demonstrating that the presence of this simple six amino acid N-terminal element enables prenylation at Rab5's C-terminus. H-Ras/Rab5 chimeras that include the conserved YXYLFK motif at the N-terminus do not become prenylated, indicating that, while this element may be necessary for prenylation of Rab proteins, it alone is not sufficient to confer properties to a heterologous protein to enable substrate recognition by the Rab geranylgeranyl transferase. Deletion analysis and studies of point mutants further reveal that the lysine residue of the YXYLFK motif is an absolute requirement to enable geranylgeranylation of Rab proteins. Functional studies support the idea that this domain is not required for guanine nucleotide binding since prenylation-defective mutants still bind GDP and are protected from protease digestion in the presence of GTP-yS. We conclude that the mechanism of Rab geranylgeranylation involves key elements of the protein's tertiary structure including a conserved N-terminal amino acid motif (YXYLFK) that incorporates a critical lysine residue.

No abstract