Tetrabrachion: a filamentous archaebacterial surface protein assembly of unusual structure and extreme stability.
The surface (S-) layer of the hyperthermophilic archaebacterium Staphylothermus marinus was isolated, dissected into separate domains by chemical and proteolytic methods, and analyzed by spectroscopic, electron microscopic and biochemical techniques. The S-layer is formed by a poorly ordered meshwork of branched, filiform morphological subunits resembling dandelion seed-heads. A morphological subunit (christened by us tetrabrachion) consists of a 70 nm long, almost perfectly straight stalk ending in four straight arms of 24 nm length that provide lateral connectivity by end-to-end contacts. At 32 nm from the branching point, tetrabrachion carries two globular particles of 10 nm diameter that have both tryptic and chymotryptic protease activity. Tetrabrachion is built by a tetramer of M(r) 92,000 polypeptides that form a parallel, four-stranded alpha-helical rod and separate at one end into four strands. These strands interact in a 1:1 stoichiometry with polypeptides of M(r) 85,000 to form the arms. The arms are composed entirely of beta-sheets. All S-layer components contain bound carbohydrates (glucose, mannose, and glucosamine) at a ratio of 38 g/100 g protein for the complete tetrabrachion-protease complex. The unique structure of tetrabrachion is reflected in an extreme thermal stability in the presence of strong denaturants (1% (w/v) SDS of 6M guanidine): the arms, which are stabilized by intramolecular disulphide bridges, melt around 115 degrees C under non-reducing conditions, whereas the stalk sustains heating up to about 130 degrees C. Complete denaturation of the stalk domain requires treatment with 70% (v/v) sulfuric acid or with fuming trifluoromethanesulfonic acid. The globular protease can be heated to 90 degrees C in 6M guanidine and to 120 degrees C in 1% SDS and represents one of the most stable proteases characterized to date.
Max-Planck-Institut für Biochemie, Martinsried, Germany.
J
J
Peters
M
M
Nitsch
B
B
Kühlmorgen
R
R
Golbik
A
A
Lupas
J
J
Kellermann
H
H
Engelhardt
J P
JP
Pfander
S
S
Müller
K
K
Goldie
D001105:Archaea / Q000378:metabolism / Q000648:ultrastructure*; D001426:Bacterial Proteins / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure*; D002462:Cell Membrane / Q000648:ultrastructure*; D005614:Freeze Fracturing; D017348:Microscopy, Electron, Scanning Transmission; D008970:Molecular Weight; D011487:Protein Conformation; D011489:Protein Denaturation
D001426:Bacterial Proteins