The cell surface glycoprotein layer of the extreme halophile Halobacterium salinarum and its relation to Haloferax volcanii: cryo-electron tomography of freeze-substituted cells and projection studies of negatively stained envelopes.
We have studied the surface layer (S-layer) of Halobacterium salinarum (formerly Halobacterium halobium), an extreme halophile requiring high concentrations of sodium, by electron microscopy of (a) isolated, negatively stained, flattened envelopes and (b) cryo-fixation of intact cells in their high-salt growth medium followed by freeze substitution and tomography of thin sections. From the negatively stained isolated envelopes we have calculated a two-dimensional, projection map that is strikingly similar to that of Haloferax volcanii, an extreme halophile requiring high concentrations of magnesium; both projection maps show the hexagonal arrangement of the morphological units with an identical center-to-center spacing of 150 A; each of the morphological units of the two species has six subunits with a similar density distribution and apparent domain organization. In contrast to the two-dimensional map, the tomographic reconstruction of Halob. salinarum does not agree in a straightforward way with the three-dimensional, electron crystallographic map of negatively stained Halof. volcanii envelopes, although the main features of the lattice and the morphological units are evident. The tomographic reconstruction of sections from epoxy-embedded material suffers from directional compression due to sectioning stress and continuous dimensional changes and mass loss due to electron irradiation. This communication consists, therefore, of three parts: (a) a comparison of the projection maps of negatively stained envelopes of Halof. volcanii and Halob. salinarum; (b) a comparison of the three-dimensional maps obtained by electron crystallography (Halof. volcanii) and low-dose cryo-tomography (Halob. salinarum); and (c) a methodological study of mass loss and dimensional changes of plastic-embedded material under low-dose conditions at room and liquid nitrogen temperatures.
Trachtenberg
S
S
Department of Membrane and Ultrastructure Research, The Hebrew University-Hadassah Medical School, Jerusalem, 91120, Israel. shlomot@cc.huji.ac.il
Pinnick
B
B
Kessel
M
M
D019843:Archaeal Proteins / Q000737:chemistry / Q000648:ultrastructure*; D002462:Cell Membrane / Q000737:chemistry / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D004583:Electrons; D017110:Freeze Substitution; D019617:Halobacterium salinarum / Q000737:chemistry / Q000648:ultrastructure*; D019622:Haloferax volcanii / Q000737:chemistry / Q000648:ultrastructure*; D007091:Image Processing, Computer-Assisted; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000648:ultrastructure*; D013045:Species Specificity; D014054:Tomography
D019843:Archaeal Proteins; D008562:Membrane Glycoproteins