The cDNA structure and expression analysis of the genes for the cysteine proteinase inhibitor cystatin C and for β₂‐microglobulin in rat brain

Tissue patterns of gene expression were analyzed by measuring mRNA levels and incorporation of radioactive amino acids for cystatin C and β₂‐microglobulin, the two extracellular proteins in the brain with the highest ratio of concentration in cerebrospinal fluid over that in blood plasma. The primary structure of rat cystatin C mRNA from choroid plexus was determined by nucleotide sequencing of cloned cDNA and the tissue patterns of gene expression were analysed by RNA blot analysis and in situ hybridization. Cystatin C was found to be composed of 120 amino acids and to contain a potential site for N‐linked glycosylation. The tissue with the highest cystatin C mRNA level was the choroid plexus of the brain. Cystatin C mRNA was also detected in lower levels in other areas of the brain, testis, epididymis, seminal vesicles, prostate, ovary, submandibular gland, and, in trace amounts, in liver. Choroid plexus pieces in culture secreted radioactive cystatin C when incubated with radioactive leucine. Rat β₂‐microglobulin cDNA was cloned and identified by nucleotide sequencing and comparison of the obtained sequence with that of mouse and human β₂‐microglobulin cDNA. Tissue levels of β₂‐microglobulin mRNA in the rat were measured by hybridization to rat β₂‐microglobulin cDNA. The highest levels of β₂‐microglobulin mRNA were observed in liver and choroid plexus. Other parts of the brain and testis contained lower levels of β₂‐microglobulin mRNA.