Human salivary cystatin S. Cloning, sequence analysis, hybridization in situ and immunocytochemistry

A human submandibular-gland (SMG) cDNA library was constructed in a lambda was constructed in a lambda gt11 Sfi-Not orientation-specific expression vector and then screened with antibody generated against human salivary cystatins. The clone C4-4 encoded an N-terminally truncated cystatin S, whereas the others encoded cystatin SN. The library was then rescreened with the C4-4, and the inserts of several positive clones were directly amplified from the eluted plaques by linear PCR and the PCR products analysed by Southern blotting and direct DNA sequencing. Two clones (C3 and C12) encoded a full-length secreted cystatin S and its leader peptide and included 5'- and 3'-untranslated regions. These clones showed a high degree of sequence similarity to cDNA clones encoding human salivary cystatin SN and genomic clones encoding cystatin SN and SA. Hybridization in situ of normal human SMG and parotid-gland (PG) tissue sections localized the cystatin-gene transcripts to the cytoplasm of serous acinar cells of both glands, with a much higher concentration of cystatin mRNA in the SMG. Immunocytochemistry localized the salivary cystatin gene products also to the serous cells, and the levels of cystatin protein correlated with the amount of cystatin mRNA, with a much stronger signal in the SMG than in the PG.