A sequence in beta-hexosaminidase from Dictyostelium discoideum required for sorting of proteins to a compartment involved in developmentally induced secretion

Subcellular distribution of "intersecting' beta-N-acetylglucosaminyltransferase in Dictyostelium discoideum. A likely marker for the Golgi apparatus

Glycoprotein processing in Dictyostelium discoideum is characterized by enzyme catalyzed steps not reported in other organisms. One of these is the formation of a beta 1 --> 4 linkage between GlcNAc and the mannose linked to the core mannose in the alpha 1 --> 6 position of N-glycosides. A simple and sensitive assay for this GlcNAc transferase activity, using a tri-mannose acceptor and a low concentration of UDP-GlcNAc, was developed. Homogenates of the organism were subjected to sub-cellular fractionation by centrifugation in discontinuous sucrose gradients. The specific activity was enriched 4-5-fold in a crude membrane fraction. The transferase was purified 10-12-fold in a membrane fraction that bands on top of 1.1 M sucrose. This fraction was also enriched in nucleotidyldiphosphatase. The enriched fraction was deficient in glucose-6-phosphatase, an endoplasmic reticulum marker. Approx. 80% of the transferase activity was latent, and unavailable to protease. Purified membranes were either subjected to phase separation in Triton X-114, or sodium carbonate extraction or sonication. In each case, the transferase behaved as an intrinsic membrane protein. Several secreted and lysosomal proteins are modified by the enzyme. These data support the idea that the GlcNAc transferase is present as an integral Golgi membrane protein and that at least the catalytic center of the transferase is on the lumenal side of the vesicles.

The primary structure of an Entamoeba histolytica beta-hexosaminidase A subunit

An Entamoeba histolytica gene (hex-A1) that encodes subunit A of the lysosomal enzyme beta-hexosaminidase has been cloned and sequenced. The inferred 59 kDa hex-A1 protein has the same molecular weight and 32% amino acid residue identity with the human and mouse proteins and 28% residue identity with the Dictyostelium protein. Northern blot analysis identified a mRNA of approximately 1.6 kb, which is in agreement with the expected size of a mRNA encoding the 522 amino acid hex-A1 protein. Southern blot analysis indicated the presence of at least two beta-hexosaminidase A subunit genes.