Three-dimensional reconstruction of maltoporin from electron microscopy and image processing.

Abstract

Two dimensional crystals of maltoporin (or phage lambda receptor) were obtained by reconstitution of purified maltoporin trimers and Escherichia coli phospholipids by detergent dialysis. Two different trimer packing forms were observed. One was hexagonal (a = 7.8 nm) and one rectangular (a = 7.8 nm, b = 13.6 nm). In this paper we describe the three‐dimensional structure of maltoporin, deduced from the study of the rectangular form by electron microscopy and image processing. At a resolution of approximately 2.5 nm, maltoporin trimers form aqueous channel triplets which appear to merge into a single outlet at the periplasmic surface of the outer membrane. The pore defined by maltoporin has a similar structure to that outlined by the matrix protein. From the results of functional studies by conductance measurement, it is concluded that the three channels defined by maltoporin act, contrary to those formed by the porin (OmpF protein), as a single conducting unit. A tentative outline of the maltoporin promoter is given. Maltoporin appears to be constituted by three different domains: a major rod‐like domain spanning the membrane, a minor domain located near the periplasmic surface of the membrane and finally a central domain responsible for the splitting of the channel.

Permeation of hydrophilic molecules through the outer membrane of gram‐negativ bacteria

“…According to this data it is very likely that OmpF [206] and LamB [208] form basically one pore with three openings faced to the surface of the cell. The three channels merge in the periplasmic half of the membrane.…”

“…The electron microscopic analysis of OmpF of E. coli B [205,2061 and Lamb [208] of E. coli K12 embedded into liposomes showed a more detailed picture of the pore-forming trimers. According to this data it is very likely that OmpF [206] and LamB [208] form basically one pore with three openings faced to the surface of the cell.…”

“…The central constriction of the OmpF pore is approximately 1 nm, which appears to be consistent with the effective diameters calculated froin the different methods described here (see Table 1). The dimensions of the LamB channel could not be given on the basis of the electron microscopic analysis [208]. So far it is not known if the pore structure (i.e.…”

Channel architecture in maltoporin: dominance studies with lamB mutations influencing maltodextrin binding provide evidence for independent selectivity filters in each subunit

“…la, d, and f, respectively. In addition, it is possible that at different depths within the plane of the membrane, pores alter their organization relative to subunits; this possibility was recently suggested by electron microscopic image enhancement studies with maltoporin trimers, which in certain packing forms are thought to constitute three independent pores at the surface but merge to form a single channel on the internal face of the outer membrane (21). However, it is not known from these studies whether the three pores at the surface were within individual subunits or at subunit interfaces.…”

Prediction of membrane‐spanning β‐strands and its application to maltoporin

“…Although the maltose-selective maltoporin (LamB) shows almost no sequence homology to the nonspecific porins from E. coli and is considerably larger, it also contains a high amount of 0-structure (Vogel & Jahnig, 1986) and has a comparable shape at low resolution (Lepault et al, 1988). A similar gross structure has also been deduced from the analysis of X-ray diffraction patterns from single crystals (Pauptit et al, 1991).…”