Examination of the protein sequence of insulin-like growth factors reveals strong heparin-binding domains (triplet of basic amino acids) that are also associated with internalization. Similar heparin internalization domains are also found on allergens and autoantigens. It was a small leap to expect that IGFs would also become autoantigens under inflammatory conditions that minimize heparan sulfate proteoglycan production.
Triplets of Basic Amino Acids Internalize Proteins
In several articles on this blog, I have discussed proteins that are internalized by their heparin binding domains. Heparin binding domains consistent only of a pair of basic amino acids, e.g. RK, flanked by one or more basic amino acids within a hydrophobic sequence of protein, are not sufficient to mediate internalization on heparan sulfate proteoglycans. A triplet of basic amino acids is usually required. Simple inspection of amino acid sequences is sufficient to identify these regions.
Internalization Triplet Identified in Insulin-like Growth Factor Binding Proteins
I noticed in a paper that insulin-like growth factors bind to epidermal growth factor receptors. I have previously written an article showing that EGF1 binds to its receptor via heparin, i.e. both the EGF and the receptor have heparin-binding domains. So I suspected that IGFs also had heparin binding domains. Inspection of the sequences readily identified simple heparin binding domains with pairs, but not triplets of basic amino acids. A search of the literature confirmed that heparin mediated IGF binding to receptors. A further search indicated that the heparin binding domains from proteins that bind and control the activity of IGFs could mediate internalization of proteins into cells and also into nuclei.Internalization Triplets Are Associated with Allergens and Autoantigens
I have previously noted that all allergens and autoantigens have internalization triplets of basic amino acids. The presence of these triplets in IGF binding proteins suggested that IGF binding proteins might also be autoantigens. A quick check of the literature showed that antibodies against IGFs themselves frequently occur in type I diabetes. This suggests that the IGF-binding protein complexes are internalized and IGFs are immunologically presented during inflammation to produce anti-IGF antibodies. It is interesting that the other autoantigens for type I diabetes, e.g. transglutaminase, also have the expected internalization triplets.
references:
Maruyama T, Murayama H, Nagata A, Shimada A, Kasuga A, Saruta T.
Anti-insulin-like growth factor-1 autoantibodies in type 1 diabetes. Ann N Y Acad Sci. 2002 Apr;958:267-70.
Miao D, Yu L, Eisenbarth GS. Role of autoantibodies in type 1 diabetes. Front Biosci. 2007 Jan 1;12:1889-98.
Goda N, Tenno T, Inomata K, Shirakawa M, Tanaka T, Hiroaki H. Intracellular protein delivery activity of peptides derived from insulin-like growth factor binding proteins 3 and 5. Exp Cell Res. 2008 Aug 1;314(13):2352-61. Epub 2008 May 29.
