Involvement of the S-layer Proteins Hpi and SlpA in the Maintenance of Cell Envelope Integrity in Deinococcus Radiodurans R1

The potential functions have been investigated of two proteins in Deinococcus radiodurans R1 predicted to be involved in the maintenance and integrity of the S layer: the hexagonally packed intermediate (Hpi) protein, and SlpA (DR2577), a homologue of an S-layer SlpA protein in Thermus thermophilus. Deletion of the hpi gene had little effect on the structure of the cell envelope or on shear- or solvent-induced stress responses. However, deletion of the slpA gene caused substantial alterations in cell envelope structure, and a significant defect in resistance to solvent and shear stresses compared to the wild-type. Ultrastructural analysis of slpA mutant cells indicated loss of much of the outer Hpi protein carbohydrate coat, the 'pink envelope', and the membrane-like backing layer. Together these results suggest that the SlpA protein may be involved in attachment of the Hpi surface layer to the inner cell envelope, and that SlpA may play an important role in the maintenance of cell envelope integrity in D. radiodurans.