S8-005 The 33 kDa subunit of photosystem II is a GTP-binding protein ¿ presence of nucleotides in the thylakoid lumen C Spetea1, T Hundal1, B Andersson1,2 1 Division of Cell Biology, Linköping University, SE-581 85 Linköping, Sweden. Fax: +46-13-2

Abstract

We have previously reported that GTP, tightly bound to the thylakoid membrane, stimulates the D1 protein degradation of photoinactivated photosystem II (PSII) (C. Spetea, T. Hundal, F. Lohmann, and B. Andersson (1999) Proc. Natl. Acad. Sci. USA 96, 6547-6552). Photolabeling with [ a-32P] 8-N3GTP of PSII core complexes and thylakoid membranes shows that the 33 kDa subunit of the oxygen-evolving complex (OEC33) is a GTP-binding protein. Saturation and competition studies show that the interaction between GTP and the OEC33 protein is specific, with an apparent Km of 22.8 µM. The binding is stimulated by light and prevented by DCMU. The OEC33 protein is a PSII component located on the lumenal side of the thylakoid membrane. So far there is no indication that the OEC33 protein may require nucleotide-binding for its function as a subunit of the oxygen-evolving complex. The discovery of GTP-binding to the OEC33 protein has two major implications. (i) It points to signal transduction events closely associated with PSII, possibly mediating the light-induced D1 protein degradation. (ii) It suggests that, against current dogmas, nucleotides are present in the thylakoid lumen. This in turn raises several questions, e.g. the presence of an unknown nucleotide transport system across the thylakoid membrane (see S24, Spetea et al., poster entitled: Nucleotides metabolism in the thylakoid lumen - implications for signal transduction across thylakoid membrane).