Sydenham's chorea is a CNS disorder and sequela of group A streptococcal infection where deposition of Abs in brain may result in movement and neuropsychiatric abnormalities. We studied human mAbs 24.3.1, 31.1.1, and 37.2.1 derived from chorea and selected for cross-reactivity with group A streptococci and brain Ags. Our novel findings reveal that Sydenham's chorea mAbs target a 55-kDa brain protein with an N-terminal amino acid sequence of MREIVHLQ corresponding to beta-tubulin. Chorea mAb specificity for purified brain tubulin was confirmed in ELISA and Western immunoblot, and significant levels of anti-tubulin IgG were found in acute chorea sera and cerebrospinal fluid. Lysoganglioside G(M1) inhibited binding of chorea mAbs to tubulin and mAb reactivity with human caudate and putamen brain sections was blocked by anti-tubulin mAb. The chorea mAbs labeled both intra- and extracellular Ags of a neuronal cell line providing evidence suggesting mimicry between intracellular brain protein tubulin and extracellular lysoganglioside. In addition, chorea mAb 24.3.1 and acute chorea sera induced calcium/calmodulin-dependent protein kinase II activity in human neuronal cells. Nucleotide sequence analysis of the chorea mAb V(H) genes revealed that mAb 24.3.1 V(H) gene was encoded by the V(H)1 germline gene family which encodes other anti-ganglioside V(H) genes associated with motor neuropathies. mAb recognition of tubulin and the neuronal cell surface with initiation of cell signaling and dopamine release supports an emerging theme in autoimmunity whereby cross-reactive or polyreactive autoantibodies against intracellular Ags recognize cell surface epitopes potentially leading to disease.