GTP-binding proteins may stimulate insulin biosynthesis in rat pancreatic islets by enhancing the signal-recognition-particle-dependent translocation of the insulin mRNA poly-/mono-some complex to the endoplasmic reticulum.
We aimed to elucidate the putative role of GTP-binding proteins in the regulation of insulin biosynthesis. For this purpose, freshly isolated rat islets were incubated in the presence of liposomes containing GDP, guanosine 5'-[beta-thio]diphosphate (GDP[S]), GTP, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), guanosine 5'-[beta gamma-methylene]triphosphate (p[CH2]ppG), guanosine 5'[beta gamma-imido]triphosphate (p[NH]ppG) and ATP, and the effects of the liposomal delivery of these substances on rates of biosynthesis of insulin and total protein were determined. Insulin biosynthesis during a 1 h incubation at 1.67 mM-glucose was stimulated by ATP- and GTP[S]-containing liposomes as compared with control liposomes. At 16.7 mM-glucose, only the GTP[S]-containing liposomes stimulated insulin biosynthesis. No inhibition of islet protein and insulin synthesis was observed with GDP-, GDP[S]-, p[CH2]ppG- and p[NH]ppG-containing liposomes. By determining the subcellular distribution of insulin mRNA, it was found that the mRNA content associated with microsomes was increased and that associated with the cytosolic mono-/poly-somes decreased when the islets were incubated with GTP[S]-containing liposomes, resulting in an approximate doubling of the ratio of microsomal to polysomal-associated insulin mRNA. ATP-containing liposomes produced no effects on the association of insulin mRNA with microsomes. By using photoaffinity labelling and immunoprecipitation techniques, specific binding of GTP[35S] to the alpha-subunit of the signal-recognition particle (SRP) receptor in islet homogenates containing physiological concentrations of GTP and GDP was demonstrated. These findings suggest that the GTP-binding subunit(s) of the SRP receptor, and possibly also of other GTP-binding proteins involved in this process, may regulate insulin biosynthesis by stimulating the translocation of insulin mRNA to the endoplasmic reticulum and by increasing preproinsulin-peptide translocation into the lumen of the reticulum.
Welsh
N
N
Department of Medical Cell Biology, Uppsala University, Sweden.
Oberg
C
C
Welsh
M
M
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000502:physiology*; D006150:Guanine Nucleotides / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D007163:Immunosorbent Techniques; D007328:Insulin / Q000096:biosynthesis* / Q000235:genetics; D007515:Islets of Langerhans / Q000378:metabolism* / Q000648:ultrastructure; D008081:Liposomes; D008297:Male; D008861:Microsomes / Q000378:metabolism; D021382:Protein Sorting Signals / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012261:Ribonucleoproteins / Q000378:metabolism*; D012270:Ribosomes / Q000378:metabolism; D018271:Signal Recognition Particle
D006150:Guanine Nucleotides; D007328:Insulin; D008081:Liposomes; D021382:Protein Sorting Signals; D012333:RNA, Messenger; D012261:Ribonucleoproteins; D018271:Signal Recognition Particle; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000255:Adenosine Triphosphate; D019204:GTP-Binding Proteins