Structural analysis of a developmentally regulated sequence encoding for a cysteine proteinase in Dictyostelium discoideum

Identification of two novel Dictyostelium discoideum cysteine proteinases that carry N-acetylglucosamine-1-P-modification

Dictyostelium discoideum makes multiple developmentally regulated lysosomal cysteine proteinases. One of these, a lysosomal enzyme called proteinase I, contains a cluster of GlcNAc-alpha-1-P-Ser residues. We call this phosphoglycosylation. To study its function, a cDNA library from vegetative cells was screened, and two novel cysteine proteinase clones were characterized (cprD and cprE). Each of them has highly conserved regions expected for cysteine proteinases, but unlike any other, each has a serine-rich domain containing three distinct motifs, poly-S, SGSQ, and SGSG. cprD and cprE cDNAs were overexpressed in Dictyostelium and the active enzymes identified. cprD codes for a protein of approximately 36 kDa (CP4), which is recognized by monoclonal antibodies against GlcNAc-1-P and fucose. cprE corresponds to a 29-kDa protein, which is recognized by antibodies against GlcNAc-1-P. mRNA for both enzymes is present in the vegetative phase and increases during growth on bacteria but decreases throughout development. When the formation of the fruiting body is complete the mRNA for both messages is detected again but in very low levels. Having cloned cDNAs for proteins that carry GlcNAc-1-P should allow us to probe the function of the carbohydrate in these putative lysosomal enzymes.

Regulation of cysteine proteinases during different pathways of differentiation in cellular slime molds

Cysteine proteinase activities have been determined using gelatin-SDS-PAGE analysis and assays based on peptide nitroanilides. Vegetative myxamoebae of all species examined contain high levels of cysteine proteinase activity present in multiple forms. In both Dictyostelium discoideum and Polysphondylium pallidum the proteinase content is dependent on whether the cells are grown axenically or in association with bacteria. In all instances development is accompanied by a decreased intracellular cysteine proteinase activity. This occurs during the formation of fruiting bodies in D. discoideum, microcysts in P. pallidum, and macrocysts in Dictyostelium mucoroides. Significant quantities of proteinase activity are always secreted by myxamoebae immediately on starvation. In D. mucoroides this leads to an almost total depletion of intracellular cysteine proteinases by the aggregation stage. As a consequence of this depletion it has been relatively easy to detect a developmentally regulated accumulation of cysteine proteinases at the enzyme activity level, something which has not yet proved possible with D. discoideum. Three cysteine proteinases are produced as D. mucoroides macrocysts develop and mature. In the case of microcyst formation in P. pallidum the proteinase contents of the developing cells and of the microcysts are dependent on how the myxamoebae are grown. In this developmental pathway at least, there is no absolute requirement for specific proteinases to be present (or absent) at a particular stage. The diversity of cysteine proteinases found in cellular slime molds and the variety of features apparent in their regulation suggest that they will prove to be very useful for investigating features of the structure/function relationships in this important group of enzymes.