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We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, α1-protease inhibitor and α2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, α1-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with Mr 51000, and then processed to two endoglycosidase-H-resistant forms having Mr 51000 and 56000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for α2u-globulin. In the cells lrcated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed α1-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized α1-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51000-Mr forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant α1-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH4Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins. [ABSTRACT FROM AUTHOR]

The protein p130 was isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-δ1 but lacking PLC activity. We show that p130 plays an important role in signaling by the type A receptor for γ-aminobutyric acid (GABA). Yeast twohybrid screening identified GABA RAP (GABAA receptor-associated protein), which is proposed to contribute to the sorting, targeting or clustering of GABAA receptors, as a protein that interacts with p130. Furthermore, p130 competitively inhibited the binding of the γ2 subunit of the GABAA receptor to GABARAP in vitro. Electrophysiological analysis revealed that the modulation of GABA-induced Cl- current by Zn2+ or diazepam, both of which act at GABAA receptors containing γ subunits, is impaired in hippocampal neurons of p130 knockout mice. Moreover, behavioral analysis revealed that motor coordination was impaired and the intraperitoneal injection of diazepam induced markedly reduced sedative and antianxiety effects in the mutant mice. These results indicate that p130 is essential for the function of GABAA receptors, especially in response to the agents acting on a γ2 subunit. [ABSTRACT FROM AUTHOR]