PRB3 null mutations result in absence of the proline-rich glycoprotein Gl and abolish Fusobacterium nucleatum interactions with saliva in vitro.

The glycosylated proline-rich glycoprotein (Gl or PRG), a product of the PRB3 gene, is a major constituent of human parotid saliva. Important functions proposed for Gl include acting as a bacterial receptor. The Gl proteins of several subjects were typed by two polyacrylamide gel electrophoresis (PAGE) systems: acid-lactate PAGE followed by staining with the periodic acid-Schiff reagent and sodium dodecyl sulfate-PAGE followed by electrophoretic transfer and staining with amido black or concanavalin A. The results showed one subject who apparently lacked Gl. The four exons, including splice junctions, for both PRB3 alleles of this subject were completely sequenced. Unexpressed (null) mutations were detected with an identical C nucleotide insertion in the same coding region of exon 3 of both alleles. This C nucleotide insertion leads to a frameshift with a premature termination codon that probably results in markedly reduced or absent PRB3 gene expression. We then used a nitrocellulose blot overlay assay to assay the bacterial receptor activity of parotid saliva from the PRB3null subject. No interactions with Fusobacterium nucleatum, shown previously to interact selectively with Gl, were detected. Together, these results suggest that this subject does not express the PRB3 gene and that one of the consequences is an altered ability to interact with a bacterium known to colonize the oral cavity.