Subcellular localization of the homocitrate synthase in Penicillium chrysogenum

There are conflicting reports regarding the cellular localization in Saccharomyces cerevisiae and filamentous fungi of homocitrate synthase, the first enzyme in the lysine biosynthetic pathway. The homocitrate synthase (HS) gene (lys1) of Penicillium chrysogenum was disrupted in three transformants (HS(-)) of the Wis 54-1255 pyrG strain. The three mutants named HS1(-), HS2(-) and HS3(-) all lacked homocitrate synthase activity and showed lysine auxotrophy, indicating that there is a single gene for homocitrate synthase in P. chrysogenum. The lys1 ORF was fused in frame to the gene for the green fluorescent protein (GFP) gene of the jellyfish Aequorea victoria. Homocitrate synthase-deficient mutants transformed with a plasmid containing the lys1-GFP fusion recovered prototrophy and showed similar levels of homocitrate synthase activity to the parental strain Wis 54-1255, indicating that the hybrid protein retains the biological function of wild-type homocitrate synthase. Immunoblotting analysis revealed that the HS-GFP fusion protein is maintained intact and does not release the GFP moiety. Fluorescence microscopy analysis of the transformants showed that homocitrate synthase was mainly located in the cytoplasm in P. chrysogenum; in S. cerevisiae the enzyme is targeted to the nucleus. The control nuclear protein StuA was properly targeted to the nucleus when the StuA (targeting domain)-GFP hybrid protein was expressed in P. chrysogenum. The difference in localization of homocitrate synthase between P. chrysogenum and S. cerevisiae suggests that this protein may play a regulatory function, in addition to its catalytic function, in S. cerevisiae but not in P. chrysogenum.