Basic proline-rich proteins of murine parotid glands: induction of mRNA by isoprenaline and post-secretion processing

Five major basic polypeptides with characteristics typical of proline-rich proteins, accumulated in parotid glands after long term isoprenaline treatment of Balb C mice. They were studied by two-dimensional gel electrophoresis and designated B1 degree, B2' degrees, B2 degrees, B3 degrees and B4 degrees on the basis of pI-dependent mobility. They were not observed in the glands of normal mice and were precipitated when glands were homogenized in 10% trichloroacetic acid unlike the three isoprenaline-induced proline-rich proteins of murine parotid glands reported previously. Isoprenaline induced six proline-rich in vitro translation products which were absent normally. Four of these species had pI-dependent mobilities almost identical to B1 degree, B2 degrees, B3 degrees and B4 degrees, indicating not only precursor/product relationships, but also that isoprenaline induced the accumulation of the proteins by regulating the mRNA. Identical salivary counterparts of the basic glandular proline-rich proteins were not detected whereas a series of smaller and more basic isoprenaline-induced polypeptides were observed in saliva (major speices B1s-B4s). The glandular proline-rich proteins were secreted from parotid tissue in vitro and the data indicate that proline-rich proteins are synthesised as precursors and processed into salivary form in the parotid glands after secretion. The relationships between the B-type in vitro translation products, parotid gland precursors and salivary proteins were also confirmed immunologically.