A 60-kilodalton immunodominant glycoprotein is essential for cell wall integrity and the maintenance of cell shape in Streptococcus mutans

Js. Chia et al., A 60-kilodalton immunodominant glycoprotein is essential for cell wall integrity and the maintenance of cell shape in Streptococcus mutans, INFEC IMMUN, 69(11), 2001, pp. 6987-6998
Citations number
Categorie Soggetti
Journal title
ISSN journal
0019-9567 → ACNP
Year of publication
6987 - 6998
SICI code
We have demonstrated previously by Western blotting that in naturally sensi tized humans, the serum or salivary antibody response to Streptococcus muta ns was directed predominantly to a protein antigen with a size of approxima tely 60-kDa. To identify this immunodominant antigen, specific serum antibo dies were eluted from immunoblots and five positive clones with inserts ran ging in length from 3 to 8 kb from identical chromosomal loci were obtained by screening a genomic expression library of Streptococcus mutans GS-5. Am ino acid sequencing established the identity of this immunodominant antigen , a 60-kDa immunodominant glycoprotein (IDG-60), to be a cell wall-associat ed general stress protein GSP-781, which was originally predicted to have a molecular mass of approximately 45 kDa based on the derived nucleotide seq uence. Discrepancy in the molecular mass was also observed in recombinant h is-tagged IDG-60 (rIDG-60) expressed from Escherichia coli. Glycosylation, consisting of sialic acid, mannose galactose, and N-acetylgalactosamine, wa s detected by lectin binding to IDG-60 in cell wall extracts from S. mutans and rIDG-60 expressed in vivo or translated in vitro. Despite the presence of multiple Asn or Ser or Thr glycosylation sites, IDG-60 was resistant to the effect of N-glycosidase F and multiple O-glycosidase molecules but not to beta -galactosidase. Insertional inactivation of the gene encoding IDG- 60, sagA, resulted in a retarded growth rate, destabilization of the cell w all, and pleiomorphic cell shape with multifold ingrowth of cell wall. In a ddition, distinct from the parental GS-5 strain, the isogenic mutant GS-51 was unable to survive the challenge of low pH and high osmotic pressure or high temperature. Expression of the wild-type gene in trans within GS-51 fr om plasmid pDL277 complemented the growth defect and restored normal cell s hape. These results suggested that IDG-60 is essential for maintaining the integrity of the cell wall and the uniformity of cell shape, both of which are indispensable for bacteria survival under stress conditions.