Phosphoinositides as key regulators of synaptic function

Synaptic localization of membrane-associated guanylate kinase-interacting protein mediated by the pleckstrin homology domain

Membrane-associated guanylate kinase-interacting protein (MAGUIN) has been identified as a protein binding postsynaptic density (PSD)-95 and synaptic scaffolding molecule (S-SCAM). MAGUIN has one sterile alpha motif, one conserved region in connector enhancer of ksr (Cnk) (CRIC), one PSD-95/Dlg-A/ZO-1 (PDZ) and one pleckstrin homology (PH) domain. There are two isoforms, MAGUIN-1 and -2. MAGUIN-1 binds the PDZ domains of PSD-95 and S-SCAM by the C-terminus, whereas MAGUIN-2 does not bind to PSD-95 or S-SCAM. Here, we have determined that MAGUIN-2 is also localized at synapses and that the synaptic localization of MAGUIN depends on the pleckstrin homology domain. The overexpressed C-terminal PDZ-binding region inhibits the synaptic targeting of PSD-95. Furthermore, the synaptic targeting of MAGUIN does not require N-methyl-d-aspartate (NMDA) receptor activity. These findings suggest that MAGUIN-1 and -2 are recruited to synapses by the PH domain and that MAGUIN-1 subsequently interacts with PSD-95 at synapses.

Regulation and recruitment of phosphatidylinositol 4-kinase on immature secretory granules is independent of ADP-ribosylation factor 1

Heterotrimeric G-proteins, as well as small GTPases of the Rho and ADP-ribosylation factor (ARF) family, are implicated in the regulation of lipid kinases, including PtdIns 4-kinases and PtdIns(4)P 5-kinases. Here, we describe a PtdIns 4-kinase activity on immature secretory granules (ISGs), regulated secretory organelles formed from the trans-Golgi network (TGN), and investigate the regulation of PtdIns4P levels on these membranes. Over 50% of the PtdIns 4-kinase activity on ISGs is inhibited by both a low concentration of adenosine and the monoclonal antibody 4C5G, a specific inhibitor of the type II PtdIns 4-kinase. Treatment of ISGs with mastoparan 7 (M7) stimulates the type II PtdIns 4-kinase via pertussis-toxin-sensitive G(i)/G(0) proteins, which, in contrast with previous results obtained with chromaffin granules [Gasman, Chasserot-Golaz, Hubert, Aunis and Bader (1998) J. Biol. Chem. 273, 16913-16920], does not require Rho A, B or C. M7 treatment also leads to an inhibition in the recruitment of ARF to ISG membranes: this inhibition is not dependent on G(i)/G(0) activation, and is not linked to the stimulation of PtdIns 4-kinase observed with M7. PtdIns 4-kinase activity on ISGs is not regulated by myristoylated ARF1-GTP, in contrast with results obtained with Golgi membranes [Godi, Pertile, Meyers, Marra, Di Tullio, Iurisci, Luini, Corda and De Matteis (1999) Nat. Cell Biol. 1, 280-287; Jones, Morris, Morgan, Kondo, Irvine and Cockcroft (2000) J. Biol. Chem. 275, 13962-13170], whereas ARF1-GTP does regulate the production of PtdIns(4,5)P(2). Our results suggest that the regulation of PtdIns 4-kinase on the ISGs differs in comparison with that on the TGN, and might be related to a specific requirement of ISG maturation.