The second b-like subunit (SBD) is a putative structural subunit of Drosophila melanogaster nicotinic acetylcholine receptors (nAChRs). Here we have produced speci®c antibodies against SBD to study, which other nAChR subunits can co-assemble with SBD in receptor complexes of the Drosophila nervous system. Immunohistochemical studies in the adult optic lobe revealed that SBD has a distribution similar to that of the a-subunit ALS in the synaptic neuropil. The subunits ALS, Da2 and SBD can be co-puri®ed by a-bungarotoxin af®nity chromatography. Moreover, anti-SBD antibodies co-precipitate ALS and Da2 and, vice versa, ALS and Da2 antibodies co-immunoprecipitate SBD protein. Two-step immunoaf®nity chromatography with immobilized antibodies against ALS and Da2 revealed the existence of nAChR complexes that include ALS, Da2 and SBD as integral components. Interestingly, the genes encoding these three subunits appear to be directly linked in the Drosophila genome at region 96 A of the third chromosome. In addition, SBD appears to be a component of a different receptor complex, which includes the ARD protein as an additional b-subunit, but neither ALS nor Da2 nor the third a-subunit Da3. These ®ndings suggest a considerable complexity of the Drosophila nicotinic receptor system. Keywords: Drosophila genome, gene cluster, immunoprecipitation, insect nervous system, neurotransmitter receptor, quaternary protein structure. Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels assembled from multiple subunits. Prototypic nicotinic receptors are found at the vertebrate neuromuscular junction where they mediate excitatory neurotransmission. These muscle receptors are pentamers of known subunit composition, i.e. a 2 bcd or a 2 bed (reviewed by Galzi et al. 1991;Hucho et al. 1996). A variety of nAChR subunits (a2±a9 and b2±b4) are expressed in the vertebrate nervous system, where they assemble into multiple subtypes of heteromeric and homomeric neuronal receptors (for review see Lindstrom 2000).In the insect CNS nAChRs are widely expressed and mediate fast excitatory synaptic transmission (Sattelle 1980). Most of these receptors are thought to constitute heteromeric complexes (Gundelfinger 1992); however, the subunit composition of native nAChRs in insects is still unknown. From the fruit fly Drosophila melanogaster, six neuronal nAChR subunits have been cloned to date; these are the a-type subunits ALS, Da2/SAD, Da3, Da4 and the b-type subunits ARD and SBD (Gundelfinger and Hess 1992;Gundelfinger and Schulz 2000;Lansdell and Millar 2000). The existence of additional nAChR subunits can be predicted based on the analysis of the Drosophila genome These authors contributed equally to this work. Abbreviations used: aBgt, a-bungarotoxin; ALS, a-like subunit; ARD, acetylcholine receptor protein of Drosophila; Da2, Drosophila a-subunit 2; Da3, Drosophila a-subunit 3; Da4, Drosophila a-subunit 4; mAb, monoclonal antibody; nAChR, nicotinic acetylcholine receptor; pAb, polyclonal antibody; SBD, second b-like ...