We have identified the first putative integral membrane pentraxin and named it neuronal pentraxin receptor (NPR). NPR is enriched by affinity chromatography on columns of a snake venom toxin, taipoxin, and columns of the taipoxin-binding proteins neuronal pentraxin 1 (NP1), neuronal pentraxin 2 (NP2), and taipoxinassociated calcium-binding protein 49 (TCBP49). The predominant form of NPR contains an putative NH 2 -terminal transmembrane domain and all forms of NPR are glycosylated. NPR has 49 and 48% amino acid identity to NP1 and NP2, respectively, and NPR message is expressed in neuronal regions that express NP1 and NP2. We suggest that NPR, NP1, NP2, and TCBP49 are involved in a pathway responsible for the transport of taipoxin into synapses and that this may represent a novel neuronal uptake pathway involved in the clearance of synaptic debris.We identified two taipoxin binding proteins for a presynaptic-acting snake venom neurotoxin, taipoxin, that blocks recycling of synaptic vesicles (1, 2). Affinity chromatography of solubilized rat brain membranes on columns of immobilized taipoxin enriches two major proteins: (i) neuronal pentraxin 1 (NP1), 1 a neuronally secreted protein with homology to serum pentraxins (2), and (ii) taipoxin-associated calcium-binding protein 49 (TCBP49), a reticular calcium-binding protein (3). NP1 has homology to previously identified pentraxins, such as serum amyloid P protein and C-reactive protein, which are elevated in the serum during acute phase response. Although the exact functions of these previously identified pentraxins are not known, they have been shown to bind, in a calcium-dependent manner, a wide variety of ligands and have been proposed to mediate the uptake of bacteria, toxins, and extracellular debris (4, 5). Homology to serum pentraxins, as well as the presence of a cleaved signal peptide and N-linked glycosylation sites, suggests that NP1 is secreted. The abundance of NP1 mRNA and rarity of NP1 protein suggest that NP1 protein has a rapid turnover. We have proposed that NP1 has a role in uptake at the synapse and that NP1 mediates the uptake of taipoxin into neurons. By low stringency screening, we identified an additional neuronal pentraxin (NP2) in human that has 54% amino acid identity with NP1 and is expressed in brain and multiple other tissues (6). Potential homologs of NP2 have been identified in guinea pig as a sperm acrosomal protein, apexin/p50 (7,8), and in rat as a neural activity-regulated pentraxin, narp (9). The second taipoxin-binding protein, TCBP49, binds calcium via six EF-hand calcium binding motifs and is localized to the lumen of reticular membranes in neurons and glia (3). It contains the carboxyl-terminal sequence HDEL which has been shown to occasionally mediate endoplasmic reticulum retention in mammalian cells (10 -12). We have suggested that NP1 binds to synaptic material and is taken up into a compartment containing TCBP49 (2, 3). We have also suggested that NP1 allows the internalization of taipoxin or a taipoxin⅐NP1 complex and t...

Neuronal pentraxin 1 (NP1), neuronal pentraxin 2 (NP2), and neuronal pentraxin receptor (NPR) are members of a new family of proteins identified through interaction with a presynaptic snake venom toxin taipoxin. We have proposed that these three neuronal pentraxins represent a novel neuronal uptake pathway that may function during synapse formation and remodeling. We have investigated the mutual interactions of these proteins by characterizing their enrichment on taipoxin affinity columns; by expressing NP1, NP2, and NPR singly and together in Chinese hamster ovary cells; and by generating mice that fail to express NP1. NP1 and NP2 are secreted, exist as higher order multimers (probably pentamers), and interact with taipoxin and taipoxinassociated calcium-binding protein 49 (TCBP49). NPR is expressed on the cell membrane and does not bind taipoxin or TCBP49 by itself, but it can form heteropentamers with NP1 and NP2 that can be released from cell membranes. This is the first demonstration of heteromultimerization of pentraxins and release of a pentraxin complex by proteolysis. These processes are likely to directly effect the localization and function of neuronal pentraxins in neuronal uptake or synapse formation and remodeling.

We identified, by affinity chromatography, two putative binding proteins for the presynaptic snake venom toxin taipoxin. We have previously characterized one of these proteins [neuronal pentraxin (NP)] as a neuronally secreted protein with homology to acute‐phase proteins. Here we report the identification of the second protein as a 49‐kDa lumenal calcium binding protein that we have named taipoxin‐associated calcium binding protein 49 (TCBP‐49). This protein contains six EF‐hand putative calcium binding domains and the carboxyl‐terminal sequence His‐Asp‐Glu‐Leu (HDEL), identical to the yeast endoplasmic reticulum retention signal. Message for this protein is present in brain, liver, muscle, heart, kidney, and testis. Antibodies to this protein label reticular organelles of neurons and glia. This localization and the specific enrichment of native and recombinant TCBP‐49 on columns of immobilized taipoxin raise the possibility that this protein interacts with internalized taipoxin, perhaps mediating its activation. The availability of pure TCBP‐49 will allow direct tests of whether TCBP‐49 alters the integrity of the oligomeric structure, phospholipase activity, or toxicity of taipoxin.

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