Cubic membranes

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Landh T. From entangled membranes to eclectic morphologies-Cubic membranes as subcehular space organizers. FEBS Lett. 1995 369 13-17. 

Aknsherqi ZA, Kohlwein SD, Deng Y. Cubic membranes a legend beyond the flatland of cell membrane organization. J. Cell Biol. 2006 173 839-844. 

Chapter 7 Cytomembranes and Cubic Membrane Systems Revisited... 

It is unfortunate that the only approach by which we can currently study cubic membrane structure is from electron microscopy, which often induces artefacts. On the other hand it is more likely that perturbations caused by EM preparation md fixation techniques are destructive to cubic membranes rather than constructive. Therefore it is unlikely that EM preparation techniques would induce the formation of cubic membranes. Indeed similar conclusions have been deduced by several authors dealing with the membrane assemblies that we identify as cubic membranes (see, e.g. [4] and [6]). Besides artefactual problems, our approach is inherently limited, because... 

It is of some importance to mention that the "waveform" pattern as defined by Gunning [8] cannot be comprehoided as a projection normal to the 221 plane of his model, which model resembles the P-PCS-structure. Among the possible candidates that we have investigated for the description of the PLB structure (the D-, P-, and G-PCS structures and four other cubic PMS s based structures), the D-PCS structure alone accounts for all structural features of the PLB that are available to us. Of particular value is the recognition of the prominent triangular-like structural elements of the D-surface, which have only been observed in the D-PCS structure. We should point out that the opposite case i.e. the lack of such triangular elements, does not exclude the D-PCS in a structural evaluation of a cubic membrane, and we have encoxmtered cases in which the aspect of the D-cubic membrane is smoothly curved. 

While the structure of these cubic membranes does represent, to date, an enigma, their functional role is an even more challenging issue since virtually nothing is known of it. Undoubtedly, structure-function relationships, so successfully applied to many other areas of science, could prove to be equally fruitful when applied to cubic membranes, as briefly addressed below. In the following sections we will highlight some of our structural findings. The outline will largely follow the order of appearance in Tables 7.1 and 7.2. 

Table 7.1. Membrane asseinblies, whose sinictutes are consistent with cubic membranes, observed in EM studies of normal cells/tissues ...

Indicates tiiat the cell/tissue was cultured. Indicates that a cubic membrane can be identified in both normal and pathogenic instances of the same cell/tissue. 

Cubic membranes in unicellular organisms prokaryotes and protozoa... 

In this group there are quite a few reports dealing with PLB-like membrane assemblies of photosynthetic lamellae, however few can be unambiguously identified as cubic membranes. One example in which we can make that identification has been observed in an aged blue-green alga [19] whose photosynthetic thylakoids were observed to continuously fold into a... 

The lattice-like membrane arrangement of the cristae of mitochondria in the giant amoeba was interpreted by Pappas and Brandt [22] to be composed of membranous tubules. If instead of discrete tubules, a continuous surface is assigned in such a way that it links the sections, a cubic membrane can be... 

The structural identification of cubic membranes does not necessarily show that cubic membranes are physiologically active structures. But there is some evidence for this in, e.g., PLB s that arrange the molecular structure of their photosynthetically active membrane. 

A curious observation is that only one of the two spaces defined by the cubic membrane seems to be involved in this process, as indicated in Fig. 7.4(b)... 

Figure 7.4 A cubic membrane in leucoplasts of root tip cells that are actively involved in "protein storage", (a) Shows two protein crystals which are encompassed by a membrane which is continuous with the cubic membrane, (b) Shows the direct continuities (arrows) between one space of the two defined by the cubic membrane and the amorphous protein-sac. From [53], reproduced with permission.

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