Sugar binding proteins of the thermoacidophile Sulfolobus solfataricus function together with ABC transporters in the uptake of sugars. They are synthesized as precursors with a class III signal peptide that are normally found in archaeal flagellins and bacterial type IV pilins. The functional expression of sugar binding proteins at the cell surface is dependent on the bindosome assembly system (Bas) that is homologous to bacterial type IV pilin assembly systems. The Bas system consists of an assembly ATPase, BasE; a membrane anchoring protein, BasF; and three small class III signal peptide containing proteins BasABC. Expression of BasEF in a S. solfataricus ΔbasEF strain restored the uptake of glucose, while an ATPase mutant of BasE was unable to complement. BasEF was detergent-extracted from S. solfataricus membranes as a stable protein complex. Solute binding proteins can be extracted from the cell surface as two high molecular mass complexes of 600 and 400 kDa, wherein the largest complex also contains the main S-layer protein SlaA. Electron microscopic analysis of the cell surface of the wild-type and ΔbasEF strain indicates that the absence of the BasEF complex causes an alteration in cell morphology and the corrugation of the S-layer pattern that is reversed by complementation with the BasEF complex. These results suggest an interaction between the S-layer and the sugar binding proteins that contribute to cell shape.
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