ATAB2 is a novel factor in the signalling pathway of light-controlled synthesis of photosystem proteins.

Plastid translational control depends to a large extent on the light conditions, and is presumably mediated by nucleus-encoded proteins acting on organelle gene expression. However, the molecular mechanisms of light signalling involved in translation are still poorly understood. We investigated the role of the Arabidopsis ortholog of Tab2, a nuclear gene specifically required for translation of the PsaB photosystem I subunit in the unicellular alga Chlamydomonas. Inactivation of ATAB2 strongly affects Arabidopsis development and thylakoid membrane biogenesis and leads to an albino phenotype. Moreover the rate of synthesis of the photosystem reaction center subunits is decreased and the association of their mRNAs with polysomes is affected. ATAB2 is a chloroplast A/U-rich RNA-binding protein that presumably functions as an activator of translation with at least two targets, one for each photosystem. During early seedling development, ATAB2 blue-light induction is lowered in photoreceptor mutants, notably in those lacking cryptochromes. Considering its role in protein synthesis and its photoreceptor-mediated expression, ATAB2 represents a novel factor in the signalling pathway of light-controlled translation of photosystem proteins during early plant development.

Isolation and structural characterization of the Ndh complex from mesophyll and bundle sheath chloroplasts of Zea mays.

Complex I (NADH: ubiquinone oxidoreductase) is the first complex in the respiratory electron transport chain. Homologs of this complex exist in bacteria, mitochondria and chloroplasts. The minimal complex I from mitochondria and bacteria contains 14 different subunits grouped into three modules: membrane, connecting, and soluble subcomplexes. The complex I homolog (NADH dehydrogenase or Ndh complex) from chloroplasts from higher plants contains genes for two out of three modules: the membrane and connecting subcomplexes. However, there is not much information about the existence of the soluble subcomplex (which is the electron input device in bacterial complex I) in the composition of the Ndh complex. Furthermore, there are contrasting reports regarding the subunit composition of the Ndh complex and its molecular mass. By using blue native (BN)/PAGE and Tricine/PAGE or colorless-native (CN)/PAGE, BN/PAGE and Tricine/PAGE, combined with mass spectrometry, we attempted to obtain more information about the plastidal Ndh complex from maize (Zea mays). Using antibodies, we detected the expression of a new ndh gene (ndhE) in mesophyll (MS) and bundle sheath (BS) chloroplasts and in ethioplasts (ET). We determined the molecular mass of the Ndh complex (550 kDa) and observed that it splits into a 300 kDa membrane subcomplex (containing NdhE) and a 250 kDa subcomplex (containing NdhH, -J and -K). The Ndh complex forms dimers at 1000-1100 kDa in both MS and BS chloroplasts. Native/PAGE of the MS and BS chloroplasts allowed us to determine that the Ndh complex contains at least 14 different subunits. The native gel electrophoresis, western blotting and mass spectrometry allowed us to identify five of the Ndh subunits. We also provide a method that allows the purification of large amounts of Ndh complex for further structural, as well as functional studies.