We have determined the nucleotide sequence of four submaxillary gland mRNAs, designated PS, SI, S2, and S3, that encode kallikrein and kallikrein-like serine proteases. The four enzymes share between 74% and 86% amino acid sequence identity and are identical in length with the exception of single two amino acid deletions in the S2 and S3 enzymes. The PS enzyme appears to be a true tissue kallikrein. The S1 enzyme shares 86% amino acid sequence homology with the PS enzyme and retains key amino acid residues thought to be primary determinants of kallikrein cleavage specificity. The S2 enzyme is rat submaxillary tonin. The amino acid sequence of the S3 enzyme is identical with tonin at 84% of its amino acid positions and retains the same amino acid substitutions at positions likely to determine substrate cleavage preferences.
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