Altered molecular size of N-acetylglucosamine 1-phosphotransferase in I-cell disease and pseudo-Hurler polydystrophy

Cell cycle-dependent phosphorylation of Sec4p controls membrane deposition during cytokinesis

The GTPase Sec4p is a critical regulator of polarized membrane traffic. Lepore et al. show that the polo-like kinase Cdc5p phosphorylates Sec4p, which promotes coordinated membrane deposition during cytokinesis.

Intracellular trafficking is an essential and conserved eukaryotic process. Rab GTPases are a family of proteins that regulate and provide specificity for discrete membrane trafficking steps by harnessing a nucleotide-bound cycle. Global proteomic screens have revealed many Rab GTPases as phosphoproteins, but the effects of this modification are not well understood. Using the Saccharomyces cerevisiae Rab GTPase Sec4p as a model, we have found that phosphorylation negatively regulates Sec4p function by disrupting the interaction with the exocyst complex via Sec15p. We demonstrate that phosphorylation of Sec4p is a cell cycle–dependent process associated with cytokinesis. Through a genomic kinase screen, we have also identified the polo-like kinase Cdc5p as a positive regulator of Sec4p phosphorylation. Sec4p spatially and temporally localizes with Cdc5p exclusively when Sec4p phosphorylation levels peak during the cell cycle, indicating Sec4p is a direct Cdc5p substrate. Our data suggest the physiological relevance of Sec4p phosphorylation is to facilitate the coordination of membrane-trafficking events during cytokinesis.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Mucolipidoses II and III variants with normal N-acetylglucosamine 1-phosphotransferase activity toward alpha-methylmannoside are due to nonallelic mutations

Normal N-acetylglucosamine 1-phosphotransferase activity toward mono- and oligosaccharide acceptor substrates was detected in cultured skin fibroblasts from mucolipidoses II and III patients who were designated as variants (one of four mucolipidosis II and three out of six mucolipidosis III patients examined). The activity toward natural lysosomal protein acceptors was absent or deficient in cell preparations from all patients with classical as well as variant forms of mucolipidoses II and III. Complementation analysis, using fused and cocultivated mutant fibroblast combinations, revealed that, while cell lines with variant mucolipidosis III constituted a complementation group distinct from that of classical forms of mucolipidoses II and III, the variant mucolipidosis II cell line belonged to the same complementation group as did the classical forms. In contrast to the mutant enzyme from variant mucolipidosis III patients that failed to recognize lysosomal proteins as the specific acceptor substrates, the activity toward alpha-methylmannoside in the variant mucolipidosis II patient could be inhibited by exogenous lysosomal enzyme preparations (bovine beta-glucuronidase and human hexosaminidase A). These findings suggest that N-acetylglucosamine 1-phosphotransferase is composed of at least two distinct polypeptides: (1) a recognition subunit that is defective in the mucolipidosis III variants and (2) a catalytic subunit that is deficient or altered in the classical forms of mucolipidoses II and III as well as in the mucolipidosis II variant.