More typically proteins like a little salt around, but that’s not always the case. And what is the pI of the protein? If it’s near 8.5 you might want to move away from that pH–maybe to pH 6.5-8.0 range somewhere. Could try going down in salt, like I said, to like maybe 50 mM and see if that helps. You could consider changing the buffer; Tris is the kind of buffer that will interact directly with proteins. You could try Bis-Tris, or HEPES, or even possibly imidazole–imidazole also can interact directly with proteins, but it’s one of those funny kind of buffers (like cacodylate): more often than not it’s either a negative or, at best, neutral factor, but when it works, it’s like magic. Less likely addivites to try when you get sufficiently desperate are things like non-ionic detergents or 5% DMSO–or some mixture of any or all of the above. How sensitive is the protein to air oxidation? Might also consider adding DTT or something similar.