Interaction of the shortest isoform of tau protein (tau3) with human 14-3-3zeta was analyzed by means of native gel electrophoresis, chemical crosslinking and size-exclusion chromatography. Phosphorylation by cAMP-dependent protein kinase (up to 2 mole of phosphate per mole of tau3) strongly enhanced interaction of tau3 with 14-3-3. Apparent K(D) of the complexes formed by phosphorylated tau3 and 14-3-3 was close to 2 microM, whereas the corresponding constant for unphosphorylated tau3 was at least 10 times higher. The stoichiometry of the complexes formed by phosphorylated tau3 and 14-3-3 was variable and was different from 1:1. 14-3-3 decreased the probability of formation of chemically crosslinked large homooligomers of phosphorylated tau3 and at the same time induced formation of crosslinked heterooligomeric complexes of tau3 and 14-3-3 with an apparent molecular mass of 120-140 kDa.