The effect of His-tag and point mutation on the activity of irisin on MC3T3-E1 cells.

Irisin is a myokine secreted from the cleavage of fibronectin type III domain-containing protein 5 (FNDC5) and has an effect on bone formation. There are limited studies about the structure of irisin and its functional unit. In order to clarify the candidate domain responsible for irisin action, we constructed several irisin variants and tested their influence on the proliferation and osteogenesis of MC3T3-E1 cells. On the one hand, His-tag was added to the N terminal or C terminal of irisin. On the other hand, the flexible region or salt bridge site were chosen as the candidate for point mutation. Alkaline phosphatase (ALP), Runt related transcription factor 2 (Runx2) and collagen type I alpha 1 (COL1α1) were chosen to test the differentiation efficiency. We found point mutation on flexible regions, Glu-57 and Ile-107, and adding His-tag on the C-terminal of irisin did affect its action. The osteogenic potential of irisin E57K, irisin I107F and irisinC-His decreased about 90.1%, 88.8% and 96.6% activity of recombinant-irisin (r-irisin) (P < 0.05), respectively. Point mutation on the salt bridge, Arg-75, partly decreased the effect of irisin (45 ± 11.3% of r-irisin) (P < 0.05). N-terminal His-tag showed almost no effect (93.5 ± 25.7% of r-irisin) (P = 0.658). This study suggested that the flexible region of residues 55-58 and 106-108, and C-terminal of irisin are vital for its activity. Disrupting the dimerization of irisin might result in a partly reduced effect on cell differentiation.