Cytochrome F Exists as Two Functionally Separate Pools in Pea Thylakoid Membranes

Abstract

The cytochrome b6/f complex (cyt b6/f) of higher plant thylakoids is pivotal in linear [1], cyclic and protonmotive forms of electron transport (see refs. [2,3], for reviews). As opposed to photosystem II and photosystem I, cyt b6/f is distributed throughout the thylakoid membrane system [4,5]. When isolated, cyt b6/f consists of four major apoenzymes, cytochrome f (cyt f), cytochrome b6 (cyt b6), the Rieske FeS protein, and “subunit 4” [6], as well as one or more small polypeptides. Ferredoxin: NADP+ oxidoreductase has also been suggested to be a functional part of the complex [7], although this question is not yet settled. The approximate stoichiometry of the isolated complex has been estimated to be 1:1:1:1 (cyt f: cyt b6: Rieske FeS protein: subunit 4) [8]. However, since this determination was made on the isolated complex, which had undergone numerous disruptive manipulations associated with its purification (e.g., high salt washes, detergent solubilization, salt precipitation and resolubilization), it is uncertain how closely this form resembles that present in vivo. Possibly, it may represent a minimal, stable form of the complex. This question becomes increasingly significant when one considers that current models of electron transport do not adequately explain the biological rationale for the ubiquitous distribution of the cyt b6/f complex in thylakoids or how its myriad of functions are regulated. To this end, we have assayed the cyt b6/f composition of thylakoid membranes with the intent of detecting possible different forms of the complex.