Neonatal rat submandibular gland protein SMG-A and parotid secretory protein are alternatively regulated members of a salivary protein multigene family

Submandibular gland-A (SMG-A) is a major secretory product of the neonatal rat submandibular gland but is not synthesized by the acinar cells of the adult gland. The leucine-rich protein is a predominant product of the adult rat parotid gland. Preliminary data had indicated that the leucine-rich protein and SMG-A might be identical proteins whose expression was differently regulated in the parotid and submandibular salivary glands. cDNA clones encoding SMG-A and the leucine-rich protein were identified by homology to a major mouse parotid gland product, parotid secretory protein (PSP), and characterized. The leucine-rich protein shares extensive sequence homology with mouse PSP throughout its 5'-untranslated, protein coding and 3'-untranslated regions, prompting our suggestion that it should henceforth be referred to as rat PSP. SMG-A is more divergent, having greatest identity with rat and mouse PSP in its signal peptide and 3'-untranslated sequences. Transcripts homologous to SMG-A and rat PSP, but more closely related to SMG-A, were identified in rat sublingual gland by Northern blot analysis. These findings indicate that rat SMG-A and PSP arise from alternatively regulated members of a multigene family.