Intrinsic Curvature Properties of Photosynthetic Proteins in Chromatophores

 

Protein-induced shaping of cellular membranes has enjoyed a great deal of interest in recent years. Common mechanisms of membrane shaping by proteins involve either protein scaffolds, e.g., BAR domains, or the shape and oligomerization of integral membrane proteins. An elegant example of membrane reshaping by membrane proteins is seen in the chromatophores of purple photosynthetic bacteria. Chromatophores are the bacterial photosynthetic pseudo-organelles which form as extensions of the cytoplasmic membrane upon transition to phototrophic growth and house the photosynthetic proteins. The shape of these chromatophores varies among species, the two most common forms being stacks of flat lamellar folds seen in Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum and small spherical vesicles in Rhodobacter (Rba.) sphaeroides and Rb. capsulatus (see Fig. 1). It is thought that the shape of the chromatophore is determined by the proteins it contains, light harvesting complexes 1 and 2 (LH1 and LH2), cytochrome bc1 (bc1), and possibly ATP synthase, and their interactions with the membrane and with each other. 

 Fig. 1 – Schematic of the vesicular photosynthetic chromatophore. Shown here is a drawing of a chromatophore from Rba. sphaeroides. The LH1-RC dimers and LH2 complexes are closely packed in the bulb of the chromatophore, as seen in AFM images, and the bc1 complex and ATP synthase, which are absent from AFM images, are tentatively placed near the neck of the chromatophore.

Picture drawn by Olga Svinarski.

In a publication in 2008, we present the first set of simulations on the three chromatophore proteins, LH2, LH1-RC-PufX dimer, and bc1, designed to study their curvature properties at the atomistic resolution. Systems of seven LH2s pack and induce curvature in a way which depends on their packing density. A single LH1-RC-PufX dimer spontaneously bends at its dimerizing interface in a way which also induces curvature. bc1 does not induce curvature in simulations, neither singly nor in pairs. These simulations are the beginning of a study of much broader scope, which seeks to explore the interactions between many light harvesting proteins that lead to the creation of a chromatophore.
Recently, the availability of a new three-dimensional density map of the LH1-RC-PufX dimer made it possible to further refine the model of the complex. Using the Molecular Dynamics Flexible Fitting method developed in the group, two aspects of the LH1-RC-PufX dimer were elucidated in two 2009 articles: the light-absorbing and subsequent energy transferring features, and the membrane-bending properties.