Mechanism of membrane invagination and shape determination in purple phototrophs

A hallmark of eukaryotes is the presence of membrane-enclosed organelles. These can take on a variety of shapes, ranging from flattened membrane sacs (as in the Golgi apparatus) to tubular networks (e.g., endoplasmic reticulum).

It is much less appreciated that a variety of bacteria also form diverse intracellular membrane structures that derive from invaginations of the cytoplasmic membrane.

We study the purple phototrophic bacteria Rhodopseudomonas palustris  and Rhodobacter capsulatus. R. palustris forms internal membrane structures that show striking morphological similarities to the Golgi apparatus. In contrast, the closely related bacterium Rhodobacter capsulatus forms spherical invaginations.

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(A) Scheme of intracellular membrane (ICM) organization in R. palustris (from Varga and Staehelin, 1983).

(B) Tomographic slice of R. palustris’  ICM.  (C) Surface-rendered model of an R. palustris cell superimposed on a tomogram slice. Outer membrane (yellow), inner membrane (blue), ICM (green) (B and C, images by Dr. Bill Tivol and Dr. Zhuo Li, Caltech).