Molecular cloning and characterization of the full-length cDNA encoding the developmentally regulated lysosomal enzyme beta-glucosidase in Dictyostelium discoideum

Abstract

The developmentally regulated Dictyostelium discoideum lysosomal enzyme beta-glucosidase is synthesized as a membrane-associated glycosylated precursor polypeptide which undergoes at least two proteolytic cleavage events to generate a soluble mature lysosomally localized protein. To begin to analyze the mechanisms regulating the sorting of this protein and the regulation during development of the expression of the encoding gene, we have cloned and sequenced a 2.6-kilobase (kb) cDNA which contains a complete 2463-nucleotide open reading frame coding for beta-glucosidase. Conceptual translation of this open reading frame predicts a polypeptide similar in molecular mass to the primary translation product of 94 kDa that also contains the same amino acid sequences of two V8 protease derived-peptides generated from the purified beta-glucosidase enzyme. The D. discoideum enzyme contained regions highly homologous at the amino acid sequence level to both bacterial and fungal beta-glucosidases, although these regions did not overlap. A potential cleavable signal sequence was also found in the first 21 amino acids followed by a highly polar stretch of 49 amino acids which (based on amino acid sequencing of the mature beta-glucosidase) represents a pro region for this protein. This region is similar in location, size, and charge to the D. discoideum alpha-mannosidase pro-I region (Schatzle, J., Bush, J., and Cardelli, J. (1992) J. Biol. Chem. 267, 4000-4007). Several small hydrophobic stretches of amino acids were also distributed throughout the protein; however, no obvious transmembrane region(s) were identified which might explain the observed membrane association of the precursor protein. Finally, Northern blot analysis indicated that the gene encoding this enzyme was under developmental regulation. The steady state level of a 2.7-kb beta-glucosidase mRNA decreased significantly during the aggregation stage of development, from high levels during growth, and then increased in the form of a larger size 2.8-kb mRNA during the final stages of development.