In vitro reconstitution of the core and peripheral light-harvesting complexes of Rhodospirillum molischianum from separately isolated components.

Abstract

In most purple bacteria, the core light-harvesting complex (LH1) differs from the peripheral light-harvesting complex (LH2) in spectral properties and amino acid sequences. In Rhodospirillum (Rs. )molischianum, however, the LH2 closely resembles the LH1 of many species in amino acid sequence identity and in some spectral properties (e.g., circular dichroism and resonance Raman). Despite these similarities to LH1, the LH2 of Rs. molischianum displays an absorption spectrum similar to the LH2 complexes of other bacteria. Moreover, its crystal structure is very similar to the LH2 of Rhodopseudomonas (Rps.) acidophila. To better understand the basis of the biochemical and spectral differences between LH1 and LH2, we isolated the alpha and beta polypeptides of the LH2 complexes from an LH2-only strain of Rhodobacter (Rb.) sphaeroides as well as the alpha and beta polypeptides from both the LH1 and LH2 complexes from Rs. molischianum. We then examined their behavior in reconstitution assays with bacteriochlorophyll (Bchl). The Rb. sphaeroides LH2 alpha and beta polypeptides were inactive in reconstitution assays, whether alone, paired with each other, or paired in hybrid assays with the complementary LH1 polypeptides of Rs. rubrum, Rb. sphaeroides, Rb. capsulatus, or Rps. viridis. The LH1 beta polypeptide of Rs. molischianum behaved similarly to the LH1 beta polypeptides of Rs. rubrum, Rb. sphaeroides, Rb. capsulatus, and Rps. viridis, forming a subunit-type complex with or without an alpha polypeptide, and forming an LH1 complex when combined with a native LH1 alpha polypeptide. Interestingly, the LH2 beta polypeptide of Rs. molischianum, in the absence of other polypeptides, also formed a subunit-type complex as well as a further red-shifted complex whose spectrum resembled the 850 nm absorbance band of LH2. In the presence of the LH1 alpha polypeptide of Rs. rubrum or Rs. molischianum, it formed an LH1-type complex, but in the presence of the LH2 alpha polypeptide of Rs. molischianum it formed an LH2 complex. This is the first reported reconstitution of an LH2 complex using only isolated LH2 polypeptides and Bchl. It is also the first example of an LH2 beta polypeptide that can form an LH1 subunit-type complex and an LH1-type complex when paired with an LH1 alpha polypeptide.