Summary: PHOTOSYSTEM 11-ASSOCIATED PROTEIN KINASE
PHOSPHORYLATES A NOVEL 6.3 kDa PROTEIN WHICH
SUBSEQUENTLY DISSOCIATES FROM PHOTOSYSTEM I1 CORE
Helen L. Race*, Jens Forsberg*, Geoffrey ~ i n d 'and John F. Alien*
*Plant Cell Biology, Lund University, S-220 07 Lund, Sweden;
Biology Dept., Brookhaven National Laboratory, Upton, NY 11973, USA.
Key words: auxiliary enzymes, kinases, protein-protein interactions, spinach.
The enzymes responsible for reversible phosphorylation of components of photosynthetic
complexesare presumed to be associated with the thylakoid membrane. Evidence
supportingthe presence of multiple protein kinases in this system is mounting.
Phosphorylation appears to be redox-regulated with activity favouredby reducing
conditions. Mild neutral detergents release the bulk of thylakoid kinases (1,2,3) but the
unsolubilisedmembranes consistentlyretain enzyme activity. This protein kinase activity
copurifies with a photosystem I1 (PS 11)core complex and catalyses phosphorylation of
intrinsicprotein components of PS I1and light harvesting complex I1(LHC 11)(4).
Characterisation of a constitutively active, photosystem 11-associatedprotein kinase (PS II-
PK) revealed it to be a distinct and novel protein with an apparent molecular mass of 58
kDa, as determined from its electrophoreticmigration (4). The enzyme activity is rapidly