The effect of nonhydrolyzable guanine nucleotides on mammalian acetyl CoA carboxylase (ACC) activity was examined. Using porous rat adipocytes and crude fat cell homogenates to study metabolic pathway flux, CMPPNP and/or GTPγS inhibited [14C] fatty acid formation by up to 95% when either [6-14C]glucose-6-phosphate or [1-14C]acetyl CoA was used as substrate. If [2-14C] malonyl CoA initiated flux, however, no inhibition was apparent. These pathway flux studies suggested that ACC was the locus of inhibition, and that the mechanism might involve a disruption of guanine nucleotide hydrolysis by the nonhydrolyzable analogues. Using partially and avidin-sepharose-purified ACC preparations from rat fat, liver and mammary tissue, citrate-stimulated CC activity was inhibited by 25-75% with 50 μM GTPγS. Related compounds and nucleotides had absent-to-minimal effects on ACC. ATPγS was inhibitory (10-30% at 5-15 μM), but always to a lesser degree than equimolar GTPγS. Filter binding assays with [α-32P]GTP or [35S] GTPγS were negative, but low-level GTPase activity was detected. Using photoaffinity labelling techniques, [α-32P]GTP was found to bind ACC and not pyruvate carboxylase. The hypothesis that citrate-responsive ACC activity may be modulated by an intrinsic or associated GTP binding site is explored. Since ACC forms polymers, as does the cytoskeletal protein β-tubulin, amino acid sequence comparisons between ACC and atypical GTP binding domain of β tubulin are presented.