Chlorophyll-protein Complexes of Higher Plants: Protein Phosphorylation and Preparation of Monoclonal Antibodies

Abstract

Since the original study by Bonaventura and Myers (1969) it has been clear that green plants have the ability to adapt to changes in the spectral quality of ambient light. In particular, plants have the ability to detect and correct an imbalance in the rates of excitation of PS I and PS II. This ability is seen clearly when plants exposed to light which preferentially stimulates PS II (‘light 2’) are suddenly exposed to excess light which preferentially stimulates PS I (‘light 1’). Initially the plants use the new light regime inefficiently, but over a period of about 5 min the quantum efficiency of photosynthesis rises as the plant alters the organization of its photosynthetic apparatus to make better use of the illumination. For about a decade it was widely believed that changes in the cationic composition of the stroma, especially Mg²⁺ ions, were solely responsible for alterations in the organization of the Ch1-protein complexes of the thylakoid (Barber, 1980). From in vitro experiments it was concluded that high salt concentrations promoted excitation energy transfer to PS II whereas low salt concentrations promoted excitation energy transfer to PS I. However, it was not established that the salt concentrations of the stroma in vivo could be lowered sufficiently to promote transfer to PS I to the extent observed, and it was also unclear how an imbalance in the rates of excitation of PS I and PS II could have the predicted effects on the ionic composition of the stroma. Yet these studies did reveal the importance of thylakoid surface charge density on the organization of Chl-protein complexes. The cation theory explained the adaptive changes in thylakoid organization in terms of the interaction between fixed surface charges on the membrane and a stroma of variable cationic composition (Barber, 1980). In the last 2–3 years a new model has emerged in which adaptive changes in Chl-protein organization are explained in terms of the reversible phosphorylation of the light-harvesting Chl a/b complex (LHC), the most abundant protein of the thylakoids of green plants (Bennett et al., 1980; Allen et al., 1981; Horton, Black, 1980; Haworth et al., 1982; Allen, 1983; Barber, 1982; Bennett, 1983). The protein phosphorylation model acknowledges the importance of stromal cations but explains the adaptive changes in terms of alterations to the surface charge of the membrane through reversible phosphorylation of a surface-exposed segment of the LHC (Bennett, 1980).