Nucleotides metabolism in the chloroplast thylakoid lumen - implications for signal transduction across thylakoid membrane Nucleotides metabolism in the chloroplast thylakoid lumen ¿ evidence for a trans-thylakoid nucleotide transporter C Spetea1,

Abstract

The unexpected finding of GTP-binding to the 33 kDa subunit of the photosystem II oxygen-evolving complex (OEC33) points to the possibility of nucleotides being present in the thylakoid lumen and their involvement in signal transduction events (see S 8, Spetea et al., poster entitled: The 33 kDa subunit of the photosystem II oxygen-evolving complex is a GTP-binding protein - presence of nucleotides in the thylakoid lumen?!). We provide in this work two lines of evidence for nucleotide-dependent reactions in the thylakoid lumen. (i) Photolabeling with [ a-32P] 8-N3GTP and [ g-32P] 8-N3ATP of spinach thylakoids indicates the existence of a 36.5 kDa nucleotide translocator in the non-appressed membrane regions. The nucleotide-binding to the putative transporter is stimulated by light, but is not affected by DCMU. (ii) Nucleoside diphosphate kinase (NDPK) activity, catalyzing the transfer of the g-phosphate group from ATP to GDP has been detected and characterized in the thylakoid lumen, and assigned to a 17.2 kDa protein. Database search indicates the presence in Arabidopsis thaliana of a nucleotide translocator and an NDPK that are predicted to be targeted to the thylakoid membrane. We propose a model in which stromal ATP is transported under certain conditions across the thylakoid membrane (by the nucleotide translocator) followed by conversion to GTP (by NDPK) in the lumen, binding to GTP-ases (OEC33) which in turn will act on various lumenal or membrane-bound effectors concomitantly with its hydrolysis.