Evidence for Multiple Functions of the Intrinsic, 32–34 Kda Chloroplast Membrane Polypeptide of Scenedesmus in Photosystem II Reactions

Abstract

A wealth of information has been developed in the past several years demonstrating that the reaction center core of photosystem II consists of minimally five major polypeptides with apparent molecular weights of 48, 44, 34, 32 and 10 kDa. These are recognized as the two apoproteins of two PS-II chlorophyll proteins, a protein of unknown, specific function, the Q_b -binding protein and cytochrome b-559 respectively. Chloroplast genes encoding for each of these components have been identified, sequenced and the amino acid sequence determined for each of these membrane components. The product of the psbA gene the 32–34 kDa intrinsic polypeptide of chloroplast membranes, has been equated with the rapidly metabolized chloroplast protein as originally discovered by Ellis and coworkers (1). It has also been described as the Q_b -binding protein, the D_b -protein and the herbicide-binding protein (2). The primary role of this protein has been deduced to be the site of binding of a plastoquinone molecule, Q_b that functions in electron transport on the reducing side of PS-II; DCMU and atrazine compete at the Q_b binding site. This interpretation is supported by the preferential labeling of a 32–34 kDa PS-II core protein by the photoaffinity label, azidoatrazine (3). A comprehensive review of this subject area is presented in (2).