PRB1, PRB2, and PRB4 coded polymorphisms among human salivary concanavalin-A binding, II-1, and Po proline-rich proteins.

Six closely linked PRP (proline-rich protein) genes code for many salivary PRPs that show frequent length and null variants. From determined protein sequences and DNA sequence analysis of variant alleles, we here report the coding and molecular basis for Con (concanavalin A-binding) and Po (parotid "o") protein polymorphisms. The Con1 glycoprotein is encoded in exon 3 of a PRB2 allele (PRB2L CON1+) with a potential N-linked glycosylation site. Because of a probable gene conversion encompassing > or = 684 bp of DNA, the "PRB2-like" Con2 glycoprotein is encoded in exon 3 of a PRB1 allele (PRB1M CON2+) with a potential glycosylation site. The PmF protein is also encoded in the PRB1M CON2+ allele, thus explaining the previously reported association between Con2 and PmF proteins. A PRB2L CON1 allele contains a single nt missense change [TCT(Ser)-->CCT (Pro)] that abolishes the potential N-linked glycosylation site (NKS-->NKP) in the Con1 protein, and this explains the Con- type. The Po protein and a glycoprotein (II-1) are encoded in the PRB4 gene, and both proteins are absent in the presence of a mutation in the PRB4M PO- allele that contains a single nt change (G--C) at the +1 invariant position of the intron 3 5'donor splice site. The genetically determined absence of the II-1 glycoprotein leads to altered in vitro binding of Streptococcus sanguis 10556 to salivary proteins, which suggests a biological consequence for null mutations of the PRB4 gene.