Identification and Characterization of G Proteins in the Mammalian Lacrimal Gland

Abstract

The regulation of protein secretion in exocrine tissues such as the lacrimal gland involves coupling of cell surface receptor activation by neurotransmitters and neuropeptides to the generation of intracellular messengers.^1,2 In lacrimal gland, components of the cyclic adenosine 3’, 5’- monophosphate (cAMP) pathway include seven transmembrane receptors whose activation leads to stimulation or inhibition of the effector enzyme adenylyl cyclase and alterations in the intracellular levels of cAMP, as well as alterations in the release of proteins into the tears.² In virtually all tissues, heterotrimeric G proteins that reversibly bind guanine nucleotides link cell surface receptor activation to adenylyl cyclase and thus are critical components of signal transduction events in the cAMP pathway. In lacrimal gland, experimental evidence for G protein-dependent regulation of adenylyl cyclase includes stimulation of the enzyme by NaF, a direct activator of Gs_α, and by guanine 5’-0–3’-thiosphosphate (GTPyS), a non-hydrolyzable analog of GTP.^3,4 Vasoactive intestinal peptide (VIP) stimulation of the enzyme is also dependent upon GTP.⁵ In the work presented here, we have used toxin-catalyzed ADP-ribosylation and immunoreaction with antisera directed against peptide sequences of the a subunits of specific G proteins to identify and characterize the G proteins present in the mammalian lacrimal gland and that mediate lacrimal secretion through transduction of extracellular signals and alterations in adenylyl cyclase activity.