Structure of the Thylakoids and Envelope Membranes of the Cyanelles of Cyanophora paradoxa

Abstract

The cyanelles of Cyanophora paradoxa Korsch. are photosynthetically active obligate endosymbionts in which phycobiliproteins serve as the major accessory pigments. Freeze-fracture electron micrographs of thylakoids in isolated cyanelies reveal long parallel rows of particles covering most of the E-face, while a more random particle arrangement is evident in some areas. The center-to-center spacing of particles within these rows is about 10 nanometers. Their mean diameter was measured at 9.4 nanometers. The particles on the P-face have a mean diameter of 7.2 nanometers. Thylakoids that retained nearly the full complement of phycobiliproteins (determined spectrophotometrically and by gel electrophoresis) were isolated from the cyanelies. In thin sections of these preparations, rows of disc-shaped phycobilisomes are evident on the surface of the thylakoids. The spacing of the rows of phycobilisomes corresponds to that of the rows of E-face particles (approximately 45 nanometers, center to center). The periodicity of the disc-shaped phycobilisomes within a row is 10 nanometers suggesting a one-to-one association between phycobilisomes and E-face particles.In addition, visualization of the protoplasmic surface (PS) of isolated thylakoids by freeze-etch electron microscopy shows that rows of discshaped phycobilisomes are aligned directly above rows of particles exhibiting two subunits, presumably the P-surface projections of the 10-nanometer intramembrane particles. These observations, together with earlier studies indicating that the 10-nanometer E-face particles probably represent photosystem II (PSII) complexes, suggest that phycobilisomes are positioned on the thylakoid surface in direct contact with PSII centers within the thylakoid membrane.The inner envelope membrane of the cyanelles, observed in freezefracture replicas, resembles cyanobacterial plasma membranes and is dissimilar to the chloroplast envelope membranes of red or green algae.The envelope of isolated cyanelies exhibits two additional layers: (a) a 5-to 7-nanometer-thick layer that lies adjacent to the inner membrane and which seems to correspond to the peptidoglycan layer of cyanobacteria; and (b) a layer external to the purported peptidoglycan layer that exhibits fracture faces similar to those of the lipopolysaccharide layer of gram negative bacteria. Our findings indicate that the supramolecular architecture of cyanelles differs only slightly from free-living cyanobacteria to which they are presumably related.Cyanobacteria, red algal chloroplasts and certain cyanobacteria- like endosymbionts, termed cyanelles, employ phycobiliproteins attached to the outer surface of their thylakoids as the major lightharvesting pigments for 02-evolving photosynthesis (3,7,10,29,35). In the atypical case of Gloeobacter violaceus, the phycobilisomes are attached to the plasma membrane (15). The cyanelles of Cyanophora paradoxa are generally regarded as cyanobacterial endosymbionts within a flagellated unicellular eukaryote (16,30,34). According to the end...

In vitro import of pre‐ferredoxin‐NADP+‐oxidoreductase from Cyanophora paradoxa into cyanelles and into pea chloroplasts

“…Inner and outer bacterial membranes are connected by a lipoprotein embedded in the outer membrane which is covalently linked to the peptidoglycan layer. This lipoprotein is missing in cyanelles, which is most probably the reason for the instability of the outer envelope membrane during isolation [23]. When isolated by conventional methods, partial loss of the outer membrane seems to be the cause for the poor performance of isolated cyanelles in CO2 fixation and protein synthesis [2], although these cyanelles appear intact upon phase contrast microscopy.…”

Structural organization of an intact phycobilisome and its association with photosystem II

“…We noted that a portion of the raw particles and two-dimensional (2D) class averages showed additional density attached to the bottom of the PBS in a position where PSII should bind [28][29][30][31][32] (Supplementary information, Figure S5A). We screened these particles and obtained a 3D reconstruction, in which we can observe a clear density with a volume in agreement with the size of a PSII dimer ( Figure 1E and Supplementary information, Figure S5B).…”

Primary structure of cyanelle peptidoglycan of Cyanophora paradoxa: a prokaryotic cell wall as part of an organelle envelope

“…Cyanophora paradoxa is the only member of this group which can be grown easily; therefore, it is the best investigated one. The round cyanelles of this obligatorily photoautotrophic protist were originally regarded as endosymbiotic cyanobacteria for morphological reasons, such as thylakoid structure (13), carboxysomes (31), and a lysozyme-sensitive peptidoglycan sacculus (37). With the onset of molecular biological investigations of C. paradoxa (18,32), it became clear that the cyanelle genome is smaller than the genomes of unicellular free-living cyanobacteria by a factor of about 25.…”