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Abstract

SNMP-1 (sensory neuron membrane protein 1) is an olfactory-specific membrane-bound protein which is homologous with the CD36 receptor family. Previous light level immunocytochemical studies suggested that SNMP-1 was localized in the dendrites and distal cell body of sex-pheromone-specific olfactory receptor neurons (ORN); these studies further suggested SNMP-1 was expressed in only one of two to three neurons in male-specific pheromone-sensitive trichoid sensilla. To better understand the expression and localization of SNMP-1, an immunocytochemical study was performed using electron microscopy to visualize the distribution of SNMP-1 among the neurons of several classes of olfactory sensilla of both male and female antennae of the silkmoth Antheraea polyphemus. SNMP-1 antigenicity was primarily restricted to the receptive dendritic membranes of ORNs of all sensilla types examined and was observed in cytosolic granules, but not plasma membranes, of the cell soma. Mean labeling densities ranged from 1 to 16 gold particles per micrometer of dendrite circumference; dendrites of trichoid and intermediate sensilla showed significantly higher labeling densities than those of basiconic sensilla. Larger dendrites of trichoid sensilla showed significantly higher mean labeling densities (13–16/µm) than smaller diameter dendrites (3–7/µm). Immunofluorescence studies using baculovirus expressed SNMP-1 and multiphoton photon laser scanning microscopy (MPLSM) indicated that rSNMP-1, which was post-translationally processed to the in vivo molecular weight, was inserted into the plasma membrane in a topography presenting extracellular epitopes. These studies suggest SNMP-1 is a common feature of the ORNs, is asymmetrically expressed among functionally distinct neurons, and possesses a topography which permits interaction with components of the extracellular sensillum lymph.