Serine Protein Phosphorylation in the Non-Appressed (Stroma) Thylakoid Membranes

Thylakoid protein phosphorylation occurs in the light (1) and in the dark (E. Garcia-Vescovi and H. Lucero, unpublished). Light phosphorylation takes place mostly at threonine residues (1), is related to the state transition mechanism (2) and is catalyzed by a 64 K protein kinase (3) that is localized in the appressed thylakoid (4) where it phosphorylates LHC II (1) and some PS II core complex proteins (5). It is accepted that a single protein kinase is involved in the light phosphorylation of thylakoid proteins since the purified 64 K kinase reconstitutes in situ the light phosphorylation pattern which is inhibited by antibodies against 64 K protein (6). We have purified from thylakoid two serine protein kinases, ChlPK 1 (25 K) and ChlPK 2 (38 K) (7,8). While ChlPK 1 was predominant, ChlPK 2 contribution was substantially smaller (7). Later we observed that ChlPK 2 activity was barely detectable or even absent in some purifications (H. Lucero, unpublished) suggesting that ChlPK 2 is sensitive to the detergent extraction. Neither ChlPK 1 nor ChlPK 2 phosphorylate LHC II (8).