Nonsense-mediated mRNA decay (NMD) is of universal biological significance1-3. It has emerged as an important global RNA, DNA and translation regulatory pathway4. By systematically sequencing 737 genes (annotated in the Vertebrate Genome Annotation database) on the human X chromosome in 250 families with X-linked mental retardation, we identified mutations in the UPF3 regulator of nonsense transcripts homolog B (yeast) (UPF3B) leading to protein truncations in three families: two with the Lujan-Fryns phenotype5,6 and one with the FG phenotype7. We also identified a missense mutation in another family with nonsyndromic mental retardation. Three mutations lead to the introduction of a premature termination codon and subsequent NMD of mutant UPF3B mRNA. Protein blot analysis using lymphoblastoid cell lines from affected individuals showed an absence of the UPF3B protein in two families. The UPF3B protein is an important component of the NMD surveillance machinery8,9. Our results directly implicate abnormalities of NMD in human disease and suggest at least partial redundancy of NMD pathways.

The discovery over five decades ago of the lysosome, as a degradative organelle and its dysfunction in lysosomal storage disorder patients, was both insightful and simple in concept. Here, we review some of the history and pathophysiology of lysosomal storage disorders to show how they have impacted on our knowledge of lysosomal biology. Although a significant amount of information has been accrued on the molecular genetics and biochemistry of lysosomal storage disorders, we still do not fully understand the mechanistic link between the storage material and disease pathogenesis. However, the accumulation of undegraded substrate(s) can disrupt other lysosomal degradation processes, vesicular traffic, and lysosomal biogenesis to evoke the diverse pathophysiology that is evident in this complex set of disorders.

The sorting of acid hydrolase precursors at the trans-Golgi network (TGN) is mediated by binding to mannose 6-phosphate receptors (MPRs) and subsequent capture of the hydrolase-MPR complexes into clathrin-coated vesicles or transport carriers (TCs) destined for delivery to endosomes. This capture depends on the function of three monomeric clathrin adaptors named GGAs. The GGAs comprise a C-terminal "ear" domain that binds a specific set of accessory proteins. Herein we show that one of these accessory proteins, p56, colocalizes and physically interacts with the three GGAs at the TGN. Moreover, overexpression of the GGAs enhances the association of p56 with the TGN, and RNA interference (RNAi)-mediated depletion of the GGAs decreases the TGN association and total levels of p56. RNAimediated depletion of p56 or the GGAs causes various degrees of missorting of the precursor of the acid hydrolase, cathepsin D. In the case of p56 depletion, this missorting correlates with decreased mobility of GGA-containing TCs. Transfection with an RNAi-resistant p56 construct, but not with a p56 construct lacking the GGA-ear-interacting motif, restores the mobility of the TCs. We conclude that p56 tightly cooperates with the GGAs in the sorting of cathepsin D to lysosomes, probably by enabling the movement of GGA-containing TCs. INTRODUCTIONThe biosynthetic transport of acid hydrolases from the Golgi apparatus to the vacuole in yeast and to lysosomes in metazoans is a multistep process that is carried out by a complex molecular machinery. Genetic approaches have led to the identification and characterization of over 70 distinct vacuolar protein sorting or Vps proteins in the yeast, Saccharomyces cerevisiae, most of which are conserved in metazoans (Bowers and Stevens, 2005). The metazoan machinery is likely to comprise additional components that are needed to generate the greater diversity of lysosome structure, dynamics and function in multicellular organisms (Mullins and Bonifacino, 2001). However, only a few of the Vps homologues and additional components in metazoans have been directly demonstrated to participate in acid hydrolase sorting to lysosomes in vivo.Among these are the transmembrane, cation-dependent (CD) and cation-independent (CI) mannose 6-phosphate receptors (MPRs) that sort newly synthesized lysosomal hydrolase precursors from the trans-Golgi network (TGN) to the endosomal-lysosomal system in mammals (Kornfeld, 1992;Ghosh et al., 2003a). This sorting begins with the binding of the hydrolase precursors, via mannose 6-phosphate groups on their N-linked oligosaccharide chains, to the luminal domains of the MPRs. The cytosolic tails of the MPRs, in turn, interact with two types of clathrin adaptor, the monomeric GGA proteins (Puertollano et al., 2001;Takatsu et al., 2001;Zhu et al., 2001) and the heterotetrameric AP1 complex (Hö ning et al., 1997;Doray et al., 2002;Ghosh and Kornfeld, 2004), leading to the concentration of the hydrolase-precursor-MPR complexes within clathrin-coated areas of the TGN (Klumperman et a...