The Ubiquitin C-Terminal Hydrolase L1 (UCH-L1) C Terminus Plays a Key Role in Protein Stability,but Its Farnesylation Is Not Required for Membrane Association in Primary Neurons

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is a deubiquitinating enzyme that is highly expressed in neurons. A possible role for UCH-L1 in neurodegeneration has been highlighted because of its presence in Lewy bodies associated with Parkinson disease and neurofibrillary tangles observed in Alzheimer disease. UCH-L1 exists in two forms in neurons, a soluble cytoplasmic form (UCH-L1^C) and a membrane-associated form (UCH-L1^M). Alzheimer brains show reduced levels of soluble UCH-L1^C correlating with the formation of UCH-L1-immunoreactive tau tangles, whereas UCH-L1^M has been implicated in α-synuclein dysfunction. Given these reports of divergent roles, we investigated the properties of UCH-L1 membrane association. Surprisingly, our results indicate that UCH-L1 does not partition to the membrane in the cultured cell lines we tested. Furthermore, in primary cultured neurons, a proportion of UCH-L1^M does partition to the membrane, but, contrary to a previous report, this does not require farnesylation. Deletion of the four C-terminal residues caused the loss of protein solubility, abrogation of substrate binding, increased cell death, and an abnormal intracellular distribution, consistent with protein dysfunction and aggregation. These data indicate that UCH-L1 is differently processed in neurons compared with clonal cell lines and that farnesylation does not account for the membrane association in neurons.     

A Novel Mechanism of Regulating the ATPase VPS4 by Its Cofactor LIP5 and the Endosomal Sorting Complex Required for Transport (ESCRT)-III Protein CHMP5

Disassembly of the endosomal sorting complex required for transport (ESCRT) machinery from biological membranes is a critical final step in cellular processes that require the ESCRT function. This reaction is catalyzed by VPS4, an AAA-ATPase whose activity is tightly regulated by a host of proteins, including LIP5 and the ESCRT-III proteins. Here, we present structural and functional analyses of molecular interactions between human VPS4, LIP5, and the ESCRT-III proteins. The N-terminal domain of LIP5 (LIP5NTD) is required for LIP5-mediated stimulation of VPS4, and the ESCRT-III protein CHMP5 strongly inhibits the stimulation. Both of these observations are distinct from what was previously described for homologous yeast proteins. The crystal structure of LIP5NTD in complex with the MIT (microtubule-interacting and transport)-interacting motifs of CHMP5 and a second ESCRT-III protein, CHMP1B, was determined at 1 Å resolution. It reveals an ESCRT-III binding induced moderate conformational change in LIP5NTD, which results from insertion of a conserved CHMP5 tyrosine residue (Tyr¹⁸²) at the core of LIP5NTD structure. Mutation of Tyr¹⁸² partially relieves the inhibition displayed by CHMP5. Together, these results suggest a novel mechanism of VPS4 regulation in metazoans, where CHMP5 functions as a negative allosteric switch to control LIP5-mediated stimulation of VPS4.     

G protein-coupled Receptor 30 (GPR30) Forms a Plasma Membrane Complex with Membrane-associated Guanylate Kinases (MAGUKs) and Protein Kinase A-anchoring Protein 5 (AKAP5) That Constitutively Inhibits cAMP Production

GPR30, or G protein-coupled estrogen receptor, is a G protein-coupled receptor reported to bind 17β-estradiol (E₂), couple to the G proteins G_s and G_i/o, and mediate non-genomic estrogenic responses. However, controversies exist regarding the receptor pharmacological profile, effector coupling, and subcellular localization. We addressed the role of the type I PDZ motif at the receptor C terminus in receptor trafficking and coupling to cAMP production in HEK293 cells and CHO cells ectopically expressing the receptor and in Madin-Darby canine kidney cells expressing the native receptor. GPR30 was localized both intracellularly and in the plasma membrane and subject to limited basal endocytosis. E₂ and G-1, reported GPR30 agonists, neither stimulated nor inhibited cAMP production through GPR30, nor did they influence receptor localization. Instead, GPR30 constitutively inhibited cAMP production stimulated by a heterologous agonist independently of G_i/o. Moreover, siRNA knockdown of native GPR30 increased cAMP production. Deletion of the receptor PDZ motif interfered with inhibition of cAMP production and increased basal receptor endocytosis. GPR30 interacted with membrane-associated guanylate kinases, including SAP97 and PSD-95, and protein kinase A-anchoring protein (AKAP) 5 in the plasma membrane in a PDZ-dependent manner. Knockdown of AKAP5 or St-Ht31 treatment, to disrupt AKAP interaction with the PKA RIIβ regulatory subunit, decreased inhibition of cAMP production, and St-Ht31 increased basal receptor endocytosis. Therefore, GPR30 forms a plasma membrane complex with a membrane-associated guanylate kinase and AKAP5, which constitutively attenuates cAMP production in response to heterologous agonists independently of G_i/o and retains receptors in the plasma membrane.