Abstract

Eukaryotic cells consist of membrane-bounded organelles which communicate with each other through vesicles for the movement of proteins and lipids between them in a process called membrane traffic. It requires different types of proteins to facilitate docking and fusion of the cargo-containing vesicles to the correct compartment. Among them are a diverse group of membrane proteins called tethering factors including multi-subunit tethering complexes (MTCs). The transport protein particle (TRAPP) complexes are a family of related MTCs that are conserved from yeast to humans. Subunits of the mammalian TRAPP complexes are known to be involved in ER-to-Golgi and intra-Golgi trafficking while playing a fundamental role in Golgi morphology. The TRAPPC11 subunit of the mammalian TRAPP III complex has been implicated in ER-to-ERGIC trafficking as well as autophagy. The mammalian TRAPP subunits are linked to a broad range of diseases of which the mechanism and the cause are not fully understood.

In this study, homozygous and compound heterozygous mutations in the TRAPPC11 gene in human fibroblasts from five individuals were characterized using several biochemical, immunofluorescence and live cell microscopy techniques to identify the defective pathways and the effect of the mutations at the cellular level. Included in this study were two individuals with GOSR2 mutations displaying similar clinical features to patients with TRAPPC11 mutations. We hypothesized that the TRAPPC11 mutations would result in a number of different defects at the cellular level given the number of pathways TRAPPC11 has been suggested to function within. The current study suggests that some of the TRAPPC11 mutations are linked to a variety of cellular phenotypes including hypoglycosylation of proteins, ER-to-Golgi trafficking defects, delay in the exit of proteins from the Golgi, Golgi fragmentation, defects in the autophagy pathway as well as partial disassembly of the complex. Golgi soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex 2 (GOSR2) is a protein located in the cis-Golgi to facilitate docking and fusion of COPII vesicles from the ER. The current study also suggests that some of the GOSR2 mutations are linked to hypoglycosylation of proteins, ER-to-Golgi trafficking defects as well as a delay in the exit of proteins from the Golgi.

The affected individuals showed novel mutations in TRAPPC11 and GOSR2 as well as mutations seen in previous studies. Some of the earlier TRAPPC11 mutations were in or near the foie gras domain, and these new TRAPPC11 mutations cluster near that region. This suggests that the foie gras domain plays a critical role in the function of the TRAPPC11 protein. Since, TRAPPC11 mutations affect the brain, eyes, liver, muscle, and bone, this suggests that the TRAPPC11 protein has a function in multiple tissue types and organs as well as homeostasis of the organism. This study and previous studies of TRAPPC11 lead to the conclusion that TRAPPC11 mutations, in general, result in neuromuscular phenotypes. In conclusion, these mutations can be added to the growing group of mutations in TRAPPC11 and GOSR2 causing neuromuscular and myopathy phenotypes. A better understanding of these mutations which result in neuromuscular phenotypes will allow for the screening of individuals who carry these mutations and further investigate the mechanism and treatment for these diseases.