Abstract
Cysteine proteases are involved in many diverse cellular processes ranging from processing of precursor proteins to intracellular degradation. In an effort to identify novel cysteine proteases, we used the polymerase chain reaction and primers directed against the catalytic sites of previously cloned cysteine proteases. From rat brain mRNA, a 600-base pair band was amplified; cloning and partial sequence analysis of this band resulted in the identification of cathepsins B and L and five novel sequences. The novel cDNAs contained a number of residues conserved in lysosomal cysteine proteases, including the active site residue His159 (papain numbering). In addition, the amino acid homology between the novel sequences and either cathepsins B, L, or H, ranged from 63 to 32%. The insert with highest homology was used to screen a rat brain cDNA library; a 1334-base pair cDNA was isolated and the nucleotide sequence determined. This sequence encodes an open reading frame of 330 amino acids which is 82% homologous to human cathepsin S, suggesting that this sequence represents rat cathepsin S. Northern blot analysis for rat cathepsin S revealed tissue-specific expression distinct from the distribution of cathepsin B and L. The regulation of expression of rat cathepsin S mRNA in response to thyroid-stimulating hormone was studied in a rat thyroid cell line FRTL-5. The level of cathepsin S mRNA was substantially increased in response to thyroid-stimulating hormone, whereas cathepsin B and cathepsin L mRNA levels were not altered by this treatment. A portion of cDNA encoding the predicted mature protein of rat cathepsin S was expressed as a glutathione S-transferase-fusion protein. The affinity-purified protein exhibited proteolytic activity with properties similar to bovine cathepsin S. Taken together, these results imply highly specific functions for cathepsin S.
Highlights
Cysteine proteasesare involved in many diverse cel- cursors torelease regulatory peptides [3]
Cysteineproteasesthatare responsiblefor limited encodes an open reading frame of 330 amino acids hydrolysis of proteins have been identified the which is 82%homologous to human cathepsin S, sug- characterization of these enzymes are lesscomplete than gesting that this sequence represents rat cathepsinS. cathepsins B, H, and L (I).Cathepsin M is identified as a Northern blot analysis for rat cathepsin S revealed proteinasethatcatalyzes hydrolysis of a neutral form of tissue-specific expressiondistinct from the distributionfructose-1,6-biphosphataseto aformwith analkaline p H
(25)) that were reported in all cysteine proteases are present in the new clones (Fig. 1).Gly148is found in all the new clones except for PCR33 where it is replaced by a Ser.All previously reported cysteine proteases contain either Ser, Gly, or Alaz9; Ser/Gly2' are present in four of the new clones, and Val is at this position in PCR31
Summary
Alignment of the amino acid sequence of a number of cysteine proteases shows two highly conserved regions When FRTL-5 cells tissues was probed for expression of cathepsin S, a 1.4-kb were treatedwithTSH,thecathepsin S mRNAincreased transcript was detected only in the brain, ovary, uterus, and about 4-fold, whereas the mRNAsfor cathepsins B and L did thyroid (Fig. 3 B ) ; expression could not be detected in liver, not (Fig. 5), suggesting the possibility that cathepsin S is kidney, or heart. These results suggest that cathepsin S is involved in thyroid hormonebiosynthesis. An arrow indicates the putative cleavage site to release the mature rat cathepsin S
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