Evidence for the concentration of alpha2u globulin by salivary glands

Comparison of the 5' upstream putative regulatory sequences of three members of the alpha 2u-globulin gene family

We have isolated and characterized seven members of the alpha 2u-globulin gene family from a rat genomic library. The 5' upstream region (up to 1250 base pairs starting from the EcoRI site in exon 2) of three clones was sequenced. The major transcriptional start points were located 25 base pairs downstream from the 'TATA' box. A very high degree of homology was observed over the entire studied region. Two of the examined genes displayed structural features which suggest that their expression may be impeded. A high degree of homology was observed between the promotor regions of alpha 2u-globulin and those of the major urinary protein (MUP) multigene family of the mouse. A remarkable feature is the variable length of an A-rich region between the putative 'CAAT' and the 'TATA' consensus sequences. The size of this region differs markedly between MUP and alpha 2u-globulin and between different members of the alpha 2u-globulin gene family. Comparison of the alpha 2u-globulin promotor with the corresponding region of other androgen-dependent genes (the C1, C2 and C3 subunits of prostatic steroid binding protein) reveals the presence of an A-rich region of homology located approximately 378 base pairs upstream from the cap site in the alpha 2u-globulin genes. This region compares well with a sequence of putative enhancer function previously demonstrated in the alpha-fetoprotein promotor and in the immunoglobulin heavy chain promoter.

Tissue-specific control of alpha 2u globulin gene expression: constitutive synthesis in the submaxillary gland

Synthesis of alpha 2u globulin, previously thought to occur only in the male rat liver, has now been demonstrated in the submaxillary salivary gland. Unlike liver, submaxillary synthesis of alpha 2u globulin mRNA is constitutive--that is, independent of the endocrine state, age and sex. Liver and submaxillary alpha 2u globulin mRNAs are of similar size, and their 5' ends map to the same region of the gene. Isoelectric focusing of in vitro translation products revealed that submaxillary mRNA encodes a more acidic subset of alpha 2u globulins than does liver. Salivary alpha 2u globulin mRNA manifests 5% nucleotide divergence, encoding 20 amino acid substitutions, which specifies a more acidic polypeptide than its hepatic counterpart. Thus the liver and submaxillary gland synthesize alpha 2u globulin from different sets of genes that are subject to very different developmental and hormonal control.

Dietary control of the renal absorption and excretion of alpha 2u-globulin

The kidneys are an important factor in controlling the excretion of the sex-dependent protein, alpha 2u globulin, in the urine. On a protein-free diet (0% casein), when the plasma level was reduced by 60%, the daily excretion of alpha 2u in the urine was reduced from a normal of 11 mg to 0.2 mg; 97% of the alpha 2u filtered by the kidneys was reabsorbed. When a normal-protein diet (20% casein) was restored, the serum alpha 2u levels were returned to normal within 48 hours. The urinary excretion, on the other hand, remained below normal until the eighth day after repletion, whereas the renal concentrations of alpha 2u reached higher than normal values by the fourth day. Thus, during protein deficiency the kidney reduced excessive losses of protein in the urine by an almost total reabsorption of alpha 2u. On a high-protein diet (50% casein), the serum levels of alpha 2u were normal, yet its excretion was elevated 80% above the normal (20 mg/day compared with 11 mg/day). To effect this increased loss, the reabsorption of the protein was reduced to 26% of the normal.