Cell biology of prokaryotic organelles

Abstract

Mounting evidence in recent years has challenged the dogma that prokaryotes are simple and undefined cells devoid of an organized subcellular architecture. In fact, proteins once thought to be the purely eukaryotic inventions, including relatives of actin and tubulin control prokaryotic cell shape, DNA segregation, and cytokinesis. Similarly, compartmentalization, commonly noted as a distinguishing feature of eukaryotic cells, is also prevalent in the prokaryotic world in the form of protein-bounded and lipid-bounded organelles. In this article we highlight some of these prokaryotic organelles and discuss the current knowledge on their ultrastructure and the molecular mechanisms of their biogenesis and maintenance.

Figure 1.

Magnetosomes can be easily visualized with various forms of electron microscopy. The electron-dense magnetite crystals are seen as a chain running through the cell in (A). Cryo-electron tomography was instrumental in demonstrating that the magnetosome membrane is an invagination of the inner cell membrane (B) and cytoskeletal filaments surround the magnetosome chain (C). (A, Reprinted, with permission from Komeili et al. 2004 [© National Academy of Sciences]; B, reprinted with permission from Komeili et al. 2006 [© AAAS]; C, image courtesy of Zhuo Li and Grant Jensen.)

Figure 2.

Photosynthetic membranes were the first of bacterial organelles to be imaged with electron microscopy. (A) is an image from a 1967 imaging study of Rhodopseudomonas palustris. The photosynthetic membranes (Th) are arranged as ribbon-like structures that are clearly continuous with the inner cell membrane (CM) at the point indicated by the arrow. These features are revealed in three dimensions in a surface rendered reconstruction of Rhodopseudomonas viridis in (B). Thylakoid membranes of cyanobacteria (C) are arranged in several circular layers and display species-specific morphologies. In contrast with photosynthetic membranes of purple bacteria, thylakoids appear to be fully separated from the inner cell membrane. (A, Reprinted, with permission, from Tauschel and Drews 1967 [© Springer]; B, reprinted, with permission, from Konorty et al. 2008 [© Elsevier]; C, reprinted, with permission, from Nevo et al. 2007 [© Nature Publishing Group].)

Figure 3.

The nucleus-like organelle of Gemmata obscuriglobus is shown in (A). The nuclear envelope (E) is a double lipid-bilayer membrane containing the chromosome (N). The inset highlights the intracytoplsmic membrane (ICM) that separates the riboplasm from the paryphoplasm (P) compartment. A simpler organization is seen in organisms such as Pirellula marina in which the intracytoplsmic membrane (ICM) differentiates the pirellulosome (PI) from the paryphoplasm (P) (B). Many of the Planctomycetes contain another unique organelle called the anammoxosome (C). Here a CET reconstruction of Brocadia fulgida is shown. The anammoxosome is the central compartment of this cell and iron particles (red) are found within it. (A, B, Reprinted, with permission, from Lindsay et al. 2001 [© Springer]; C, reprinted, with permission, from Niftrik et al. 2008a [© ASM].)

Figure 4.

Chlorosomes of Chlorobium tepidum appear as flattened ovals arranged around the cell periphery (A). A representation of a single carboxysome based on CET imaging. The interior of the carboxysome appears to be packed with RuBisCO based on similarities between the known crystal structure of the enzyme and electron-dense entities seen in CET reconstructions (B). A TEM image of ta cyanobacterial cell reveals that the cytoplasmic space is filled with gas vesicles sectioned in two different orientations (C). (A, Reprinted, with permission, from Frigaard et al. 2002 [© ASM]; B, reprinted, with permission, from Iancu et al. 2007 [© Elsevier]; C, reprinted, with permission, from Walsby 1994 [©ASM].)