A CARBOXYL-TERMINAL LEUCINE-ZIPPER IS REQUIRED FOR TYROSINE-HYDROXYLASE TETRAMER FORMATION

Tyrosine hydroxylase catalyzes the rate-limiting reaction in the biosy nthesis of the catecholamine neurotransmitters and hormones (dopamine, norepinephrine, and epinephrine). Rat tyrosine hydroxylase exists, in its native form, as a tetramer composed of identical 498 amino acid s ubunits. There is currently no information describing the molecular in teractions by which the four monomeric tyrosine hydroxylase subunits a ssemble into an active tetramer. Mutational analysis was performed on bacterially expressed enzyme to assess the role of a putative C-termin al leucine zipper in the assembly of subunits into the tetrameric holo enzyme. Deletion of the C-terminal 19 amino acids, or mutation of a le ucine residue (to an alanine), converts the enzyme from a tetrameric t o a dimeric form that exhibits greater structural heterogeneity. This change in macromolecular form is accompanied by a 75% (deletion mutati on) to 20% (Leu --> Ala mutation) reduction in specific activity of th e enzyme. This represents the first report of the functional involveme nt of a region containing a leucine zipper motif in the assembly and a ctivity of a neuronal enzyme.