So far three splice variants of the a:o-gen¢ coding for two qo proteins have been identified by molecular cloning, and the corresponding proteins purified, In the preen( study subtype-specific peptide antibodies revealed the existence of an electrophoretically distinct third form of ~o in mammalian brain membranes which migrates more slowly on SDS-PAGE and shows a more acidic pl value than the other ~to-subtmits. Each of the three ao-subunits is detected as two isol'orms when enriched from brain membranes, Rodent ao-subunits differ from non-rodent species in their electrophoretic mobilities, The results suggest that (it there may exist a novel go-subunit which reacts with an aot-subunit-slX'Cific antibody, (ii) each %-subunit may exist in more than one co-or posttran~lationally modified iso form in brain membranes, and (iii)differences betw~n ao-subunit..~ from different species exist which are detectable by gel electrophorctic methods.