Formation of Two-Dimensional Crystals of Membrane-Anchored and Water-Soluble Proteins

Abstract

The formation of two-dimensional (2-D) crystals of biological macromolecules is of interest for nanotechnological applications. Protein 2-D crystals may be used as molecular sieves and/or support devices as components of biosensors etc. [1]. Functionally specialized 2-D crystals, containing transport or catalytic proteins, provide a certain function in a highly efficient and vectorial manner. Future developments may allow the design of more complex structures such as multilayers made from different proteins or arrays of functionally linked oligo- or multimeric complexes consisting of multiple protein species [2]. Regular 2-D arrays, either truely crystalline or densely packed molecules, are one of the basic structures taht might be used to construct more sophisticated protein-based devices. 2-D crystals have some interesting features:

Properties of a Cyclodextrin-specific, Unusual Porin from Klebsiella oxytoca

“…This observation and the fact that the odd-numbered reflections with the indices h0 and 0k in the power spectrum were usually very weak or completely missing, are compatible with the properties of the two-sided plane group c12 and a unit cell containing four CymA molecules or asymmetric structural units, respectively (23). The averaged unit cells show stainfilled channels or cavities that are not arranged in trimers, being typical for other bacterial porins (42,43), but occur in pairs representing molecules in upside-down orientation with respect to each other (central pair in the unit cell shown in Fig. 5).…”

“…Although the effective channel size cannot be extracted from projection images for fundamental reasons, it is obvious from the averages that the stain-filled regions revealed by electron microscopy possess a diameter of 1.2-1.4 nm, which is somewhat less than the apparent width of a funnel-shaped porin pore in electron micrographs (42,43). However, this value is close to the size expected to be required for the CDs and might, thus, represent a reasonable estimate for the channel diameter.…”

Crystal structure of Omp32, the anion-selective porin from Comamonas acidovorans, in complex with a periplasmic peptide at 2.1 Å resolution

“…CF2 crystals correspond to crystals of type I [29] as they contain lateral contacts between adjacent proteins of the same layer. The lateral arrangement reflects the structure of two-dimensional crystals of several membrane proteins including Omp32 [31,32]. The smallest lateral protein-toprotein distance in CF2 crystals is 4.5 Å and may be limited by the repelling forces of several charged lysine residues in the surface loops (see below).…”

Characterization of a porin from the outer membrane of Vibrio anguillarum

“…The architecture of the trimeric molecular unit and its dimensions (diameter, Ϸ6.5 nm) are strongly reminiscent of those of the pore-forming protein complexes in the OMs of bacteria, i.e., the porins (17,21). The heavily stain-filled areas represent the triplets of pores, the bright structures surrounding the pores originate mainly from the additional stain-excluding protein portions outside the membrane plane, and the greyish areas between pore triplets apparently represent the surface of the lipid (Fig.…”

“…In contrast, Bordetella pertussis (28) and Thermotoga maritima (42) possess regularly structured membranes because of the native 2D crystallinity of the porins. In order to identify the structural nature of the V. anguillarum MOMP by means of electron microscopy, it was necessary to reconstitute the purified protein in artificial membranes in a (17,21) and other membrane-bound (27) or membrane-anchored (39) surface proteins. But the V. anguillarum MOMP appeared to cluster even in presence of detergent, as the conductance step analysis suggested, which may reflect the protein's inherent propensity to form regular aggregates.…”