The unconventional secretion of ARMS2.
Age-related maculopathy susceptibility 2 (ARMS2) is a small (11 kDa), primate-specific protein found in the extracellular matrix of the choroid layer in the eye. Variants in the corresponding genetic locus are highly associated with age-related macular degeneration, a leading cause of blindness in the elderly. So far, the physiological function of ARMS2 has remained enigmatic. It has been demonstrated that ARMS2 is a genuine secreted protein devoid of an N-terminal leader sequence, yet the mechanism how it exits the cells and enters the choroidal matrix is not understood. Here, we show that ARMS2 efficiently recruits lectin chaperones from the cytosol and colocalizes with calnexin-positive and protein disulfide isomerase-negative vesicle-like structures. Site-directed mutagenesis revealed critical elements for this interaction. Mutant forms proving unable to interact with the calnexin/calreticulin system failed secretion. On the other hand, blocking the endoplasmic reticulum to Golgi transport with brefeldin A had no effect on ARMS2 secretion. As we found ARMS2 colocalizing with GRASP65, a marker for unconventional protein secretion, autophagic factors are likely to be key in its export. Interleukin-1ß (IL-1ß) is the most established example of secretory autophagy. Co-expression experiments, however, suggest that the transport of ARMS2 is different from that of IL-1ß. In conclusion, in this work we show that ARMS2 is externalized via an unconventional pathway bypassing Golgi. Its intracellular separation from the classical secretion pathway suggests that the maturation of the protein requires a specific biochemical niche and/or may be needed to impede the premature formation of unwanted protein-protein interactions.
Center for Ophthalmology, Institute of Ophthalmic Research, University of Tübingen, Röntgenweg 11, D-72076 Tübingen, Germany and eloed.koertvely@uni-tuebingen.de.
Elod
E
Kortvely
Department of Protein Science, Helmholtz Zentrum München - German Research Center for Environmental Health, Ingolstädter Landstr. 1, D-85764 Neuherberg, Germany.
Stefanie M
SM
Hauck
Department of Protein Science, Helmholtz Zentrum München - German Research Center for Environmental Health, Ingolstädter Landstr. 1, D-85764 Neuherberg, Germany.
Jennifer
J
Behler
Center for Ophthalmology, Institute of Ophthalmic Research, University of Tübingen, Röntgenweg 11, D-72076 Tübingen, Germany and.
Nurulain
N
Ho
Center for Ophthalmology, Institute of Ophthalmic Research, University of Tübingen, Röntgenweg 11, D-72076 Tübingen, Germany and.
Marius
M
Ueffing
D020126:Brefeldin A / Q000494:pharmacology; D006056:Golgi Apparatus / Q000235:genetics / Q000378:metabolism*; D000077614:Golgi Matrix Proteins; D057809:HEK293 Cells; D006801:Humans; D053583:Interleukin-1beta / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011506:Proteins / Q000235:genetics / Q000378:metabolism*
C514392:ARMS2 protein, human; C108705:GORASP1 protein, human; D000077614:Golgi Matrix Proteins; C508585:IL1B protein, human; D053583:Interleukin-1beta; D008565:Membrane Proteins; D011506:Proteins; D020126:Brefeldin A