The use of ubiquitin-peptide extensions as protein kinase substrates

Identification of the amino acids on a neuronal glutamate receptor recognized by an autoantibody from a patient with paraneoplastic syndrome

Autoantibodies from a patient with paraneoplastic disease were identified previously to bind to the glutamate receptor (GluR) subunit GluR5 and to function as potential allosteric modulators of receptor activity (Gahring et al. [1995] Mol Med 1:245-253). In the present study we have used deletion mapping and mutagenesis to define the residues in GluR5 bound by this autoreactivity. The autoantibody contact residues include residues K497, N508, K510, E512, and to a lesser extent Q507. Residues 507-512 confer autoantibody specificity of the autoreactivity to GluR5. These residues have been shown in crystallographic studies (Armstrong et al. [1998] Nature 395:913-917) to participate in a loop structure, whereas residue K497 is located on a beta-strand. Notably, this binding spans tyrosine 504, a residue important in forming the agonist-binding site. We propose that autoantibody binding of essential residues in this GluR5 autoantigenic region defines a subunit-specific allosteric regulatory site on neuronal glutamate receptors and suggests how receptor dysfunction and region-specific neuronal death in the brain can progress in certain autoimmune neurological diseases.

Identification of amino acids in the glutamate receptor, GluR3, important for antibody-binding and receptor-specific activation

We reported (Twyman, R. E., Gahring, L. C., Speiss, J., and Rogers, S. W. (1995) Neuron 14, 755-762) that antibodies to a subregion of the glutamate receptor (GluR) subunit GluR3 termed GluR3B (amino acids 372-395), act as highly specific GluR agonists. In this study we produced additional rabbit anti-GluR3B-specific antibodies, ranked them according to their ability to function as GluR agonists and characterized the immunoreactivity using deletion and alanine substitution mutagenesis. These anti-GluR3B antibodies bound to a subset of the residues in GluR3B (amino acids 372-386), of which glutamate 375, valine 378, proline 379, and phenylalanine (Phe) 380 were preferred. The level of GluR activation correlated with the binding of antibody to Phe-380, which suggests that immunoreactivity directed toward Phe-380 is an index for the anti-GluR agonist potential. Since the identity of this residue varies between respective GluR subunits, this suggested that this residue may be important for imparting antibody subunit specificity. To test this possibility, the alanine in GluR1 was converted to a phenylalanine, which extended the subunit specificity from GluR3 to the modified GluR1. We conclude that antibody contacts with key residues in the GluR3B region define a novel GluR subunit-specific agonist binding site and impart subunit-specific immunoreactivity.

Inhibition of ubiquitin-mediated proteolysis by the Arabidopsis 26 S protease subunit S5a

A variety of protease inhibitors have been used to study ubiquitin-dependent proteolysis by the 26 S protease. However, these inhibitors lack complete specificity and thus affect ubiquitin-independent pathways as well. We recently identified an Arabidopsis protein, MBP1, that is homologous to subunit 5a (S5a) of the human 26 S protease complex. MBP1 and S5a bind multiubiquitin chains with high affinity and presumably facilitate the recognition of ubiquitin conjugates by the 26 S protease. We show here that free MBP1 can be a potent inhibitor of ubiquitin-dependent proteolysis in several cell-free systems. When added to reticulocyte lysates or to Xenopus egg extracts, the plant protein effectively blocked the degradation of multiubiquitinated lysozyme and cyclin B, respectively. MBP1 did not enhance the removal of ubiquitin from lysozyme or affect the ability of the 26 S complex to hydrolyze fluorogenic peptides. These data suggest that the plant protein specifically interferes with the recognition of ubiquitin conjugates by the 26 S protease. Thus MBP1, human S5a, and their homologs should prove to be valuable reagents for investigating cellular events mediated by ubiquitin-dependent proteolysis.

A proteasome activator subunit binds calcium

We recently cloned a cDNA encoding the 29-kDa subunit of human red blood cell regulator (REG), a potent activator of the multicatalytic protease (Realini, C., Dubiel, W., Pratt, G., Ferrell, K., and Rechsteiner, M. (1994) J. Biol. Chem. 269, 20727-20732). The sequence of this subunit contains 28 "alternating" lysine and glutamic acid residues (a KEKE motif). Similar regions are present in a number of Ca(2+)-binding proteins, and using standard filter assays, the recombinant protein is shown to bind 45Ca2+ and ruthenium red. 45Ca2+ is also bound to a ubiquitin extension protein containing the 28-residue KEKE region from the 29-kDa REG subunit. Thus, the 29-kDa REG subunit is a Ca(2+)-binding protein, and its KEKE region is able to bind divalent cations. Ca2+ reversibly inhibits the enhanced peptidase activity of complexes between the multicatalytic protease and recombinant REG. This raises the possibility that multicatalytic protease activity is regulated by calcium in vivo.

Ubiquitin-RAS peptide extensions as substrates for farnesyl-protein transferase and carboxymethyltransferase

Using oligonucleotide-mediated 'loop-in' mutagenesis strategies in M13, a heat-inducible ubiquitin (Ub) gene was extended by sequences coding for the C-terminal 11 amino acids of Ha-RAS. The resulting gene was transformed into AR13 and production of the Ub-peptide extension was induced by heat treatment. After one-step purification, the fusion protein (Ub-cRAS) was used as a substrate for farnesyl-protein transferase. Ub-cRAS was farnesylated on incubation in Xenopus egg extract or rabbit reticulocyte lysate. In contrast, when serine was substituted for the last cysteine in the RAS extension, transfer of the [3H]farnesyl group from [3H] farnesyl pyrophosphate to the modified Ub-cRAS was not observed. Farnesylation of Ub-cRAS permitted us to develop an easy membrane-binding assay for farnesyl-protein transferase enzyme activity. Using this assay, we partially purified the enzyme from rabbit reticulocyte lysate. We also detected methylation of the farnesylated Ub-cRAS terminus in Xenopus egg extract.

Evidence indicating that the multicatalytic proteinase of rabbit reticulocytes is not incorporated as a core enzyme into a 26 S proteinase complex

We have reinvestigated the recent proposal that the multicatalytic proteinase, together with other components of reticulocyte lysate, may become incorporated into a very large, "26 S" proteinase complex via an ATP-dependent process. Different from these published results, we consistently isolate the multicatalytic proteinase as a 650,000 Da "20 S" multisubunit proteinase. Analysis on nondenaturing polyacrylamide gels of reticulocyte fractions containing the putative complexed form of the multicatalytic proteinase reveal that activity against succinyl-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin is associated with two groups of protein of different molecular mass. One migrates like multicatalytic proteinase purified to homogeneity, displays, on sodium dodecyl sulfate gels, a set of protein species in the range of 23,000-32,000 Da, characteristic of the multicatalytic proteinase, and is recognized by a monospecific antibody to the enzyme. In contrast, the activity associated with the higher molecular mass (26 S) proteinase complex lacks the typical multicatalytic proteinase subunits and is devoid of antigenic material, when tested with the antibody. These results confirm and extend our recent findings in mouse liver by showing that the multicatalytic proteinase is not a constituent of a 26 S proteinase complex.