1988 Volume 104 Issue 5 Pages 801-806
cDNA clones of the mRNA for rat liver carboxyesterase El, one of thecarboxyesterases exclusively located on the luminal side of microsomal vesicles, were isolated. Sequence analysis of 2 kbp long cDNA revealed the primary structure of carboxyesterase El, which consisted of 549 amino acids (Mr 60, 171.71) and contained an extra peptide of 18 amino acids at the NH2, -terminus of the mature enzyme. Comparison of the deduced primary structure and sequences of some proteolytic fragments of the purified enzyme indicated the multiplicity of the enzyme. The extra peptide at the NH2-terminal had features in common with the signal peptides of most secretory proteins. However, no polar amino acid residues existed before the hydrophobic core of the signal peptide. A new interpretation is proposed to explain how the signal peptide without the NH2-terminal polar residues works. A tetrapeptide (KDEL) which was shown to keep a few microsomal proteins in the lumen of the endoplasmic reticulum was not found in the primary structure of carboxyesterase El, which suggested the existence of another mechanism for retention of proteins in thelumen of endoplasmic reticulum. Carboxyesterase El showed significant homology with the COOH-terminal portion of thyroglobulin.
