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E. Kortvely, S. M. Hauck, M. Blindert, K. Elisabeth, M. Ueffing; An Alternative Signal Peptide Motif Tags ARMS2 for Secretion. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4115.
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© ARVO (1962-2015); The Authors (2016-present)
ARMS2 represents a major susceptibility locus for AMD; with its risk-variant possibly contributing up to >30% of all AMD cases. The biological function of this intriguing protein, however, has remained elusive so far. We have recently shown that ARMS2 interacts with components of the extracellular matrix (Kortvely et al., IOVS 2009), where it is likely to act within a protein network of extracellular proteins involving also Fibulin-1, Fibulin-6, TIMP3 (tissue inhibitor of metalloproteinase 3), and MMP-9 (matrix metalloproteinase 9). Although ARMS2 lacks a classical secretion signal sequence, translocation to the ER is a prerequisite for secretion. To gain an understanding of the mechanism responsible for the extracellular targeting of ARMS2, this alternative secretory route was further dissected.
Cassette mutagenesis was used to generate a series of plasmids coding for mutated ARMS2, where single amino acids in the C-terminal region of the protein were substituted with alanine (alanine screen). The localizations of the mutated ARMS2 proteins in transfected cells were revealed by immunocytochemistry using our own ARMS2-specific monoclonal antibodies. A blast search within the human protein database was undertaken, in order to identify further proteins possessing the same putative targeting signal.
Within the last eight amino acids (-SIIHTAAR*) encoded by the second exon of ARMS2, the presence of the two neighboring isoleucines is indispensable for proper targeting. The substitution of the other amino acids did not result in subcellular localization distinguishable from the wild-type situation. The same C-terminal double isoleucine motif was found in further 64 human proteins. This subset of human proteins also includes TIMP-3, a protein implicated in Sorsby's fundus dystrophy. This common C-terminal motif may be used to commit the proteins to secretion.
The secretion of ARMS2 suggests an alternative secretory targeting of the protein, depending on a C-terminal signal sequence and lacking a classical, N-terminal signal peptide. Because ARMS2 localizes primarily at sites corresponding to drusen deposition, this secretory mechanism may play an important role in the pathogenesis of AMD.
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