Epsilon Open Archive

Abstract

Nucleoside Diphosphate Kinase (NDPK) catalyses the transfer of a phosphate from nucleoside triphosphates to a nucleoside diphosphate, is ubiquitously found in all organisms from bacteria to human. It was discovered that the genes nm23 and awd, which encode NDPKs are involved in tumour metastasis and Drosophila development, respectively. Thus, NDPK isoforms have been suggested to have specific regulatory functions in addition to their catalytic activity. Plant NDPKs are also involved in a number of intracellular signalling events such as phytochrome A response, UV-B light signalling and heat shock response. The main focus in this thesis concerns the role of the plant (Pisum sativum L. cv Oregon sugarpod) mitochondrial NDPK3 isoform.

The NDPK3 is localized to both the intermembrane space and to the mitochondrial inner membrane. The membrane bound NDPK3 is firmly attached to the membrane through the ATP/ADP translocase. The ATP/ADP translocase and NDPK3 complex may be a part of the contact sites for channelling metabolites from mitochondria to cytosol and vice versa.

NDPK3 was shown to be dually targeted to both mitochondria and chloroplasts where the major amount of the protein is found in mitochondria. The protein was crystallized and the first X-ray structure of a plant NDPK is reported. In agreement with other eukaryotic NDPKs, the plant enzyme is a hexamer. Two conserved serine residues, S119 and S69 involved in serine autophosphorylation and oligomerization, respectively, was identified. We show that the Ser autophosphorylation depends on enzyme activity. The mutation of S69 to Ala decreased the enzymatic activity dramatically. Changes in the oligomeric pattern of S69A were observed. Thus, the S69 residue is important for the stabilization of the oligomeric state of NDPK3.

Adenylate Kinase was identified as an interacting partner of the IMS located NDPK3. The interaction modulates the activity of the enzymes where Adenylate Kinase stimulates NDPK3 and NDPK3 inhibits Adenylate Kinase with unchanged ADP production as an outcome. Cyclic AMP (cAMP) and calcium inhibit the activity of both NDPK3 and Adenylate Kinase. This is a novel regulatory relationship between cAMP and calcium signalling and nucleotide metabolism mediated by NDPK3 and Adenylate Kinase and their interaction.