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Abstract:
The cell walls of Sulfolobales species consist of proteinaceous
S-layers assembled from two polypeptides,
SlaA and SlaB. We isolated the large S-layer
protein of Acidianus ambivalens and both S-layer
subunits of Sulfolobus solfataricus and Metallosphaera
sedula, respectively. The slaAB genes,
lying adjacently in the chromosomes, are constitutively
transcribed as bicistronic operons in
A. ambivalens and S. solfataricus. A smaller slaA
transcript appeared in Northern hybridizations of
A. ambivalens RNA. PCRs experiments showed that
80–85% of the transcripts stop at an oligo-T terminator
downstream of slaA while 15–20% are read
through to a second terminator downstream of slaB.
The bicistronic operons including promoter and terminator
regions are conserved in the Sulfolobales.
While no SlaA homologue is found outside the Sulfolobales,
SlaB is distantly similar to S-layer proteins
of other Crenarchaeota, e.g. the Staphylothermus
marinus tetrabrachion. Molecular modelling suggests
SlaBs to be composed of 2–3 consecutive beta sandwich
domains, a coiled-coil domain of 15–17 nm in
length and a C-terminal transmembrane helix. Electron
microscopy shows crystalline protein arrays with
triangular and hexagonal pores. We propose that the
mushroom-shaped ‘unit cells’ of the Sulfolobales’
S-layers consist of three SlaBs anchoring the
complex in the membrane and six SlaAs forming the
detergent-resistant outer sacculus.
