An unusual glycoform of human salivary mucin MG2

Since in a previous study we encountered a subject with an unusual split MG2 banding pattern, the aim of this study was to investigate the molecular basis of this observation. Submandibular/sublingual secretion was collected under resting and stimulated conditions and examined on Western blots probed with anti-MG2 antibodies or on gels stained with periodic acid-Schiff reagent. Genomic DNA was isolated and the N-, tandem repeat (TR), and C-terminal regions of MUC7 were amplified by PCR since MG2 is known to display a genetic polymorphism. Although the typical appearance of MG2 on blots and gels is a single 180 kDa band, salivary secretions from the subject exhibited doublet immunoreactive bands of approximately 180 and 125 kDa. Additionally, under resting conditions the 180 kDa band was predominant whereas upon stimulation the 125 kDa band became predominant. Genomic DNA analysis showed that MUC7 in the individual with split MG2 bands was not truncated and that the MUC7 genotype in this individual was (6/6) where both alleles encoded six TRs. The MG2 split banding pattern observed in this subject was not derived from proteolytic degradation of this salivary mucin in whole saliva or from genetic polymorphism. The expression of two isoforms of MG2 could in principle improve or reduce the activity of this key component of the oral host defense system.