This addresses the general case, rather than the particular case of the insulin receptor, which is answered by @canadianer.
The General Problem of unexpected ligands in PDB structures
In preparing a relation database of about 400 protein structures I encountered the problem of distinguishing ligands that were substrates or cofactors of the enzyme from those that were not. And, believe me, the latter were widespread (and a real pain). In some cases it was obvious that the ‘ligand’ was the buffer (often MES) in which the protein was dissolved or ions such as sulphate. In the case of sugars, I assumed that the latter were present to aid crystalization. That view seems to be supported — in some cases at least — by a review by McPherson and Gavira. In a list of eight categories of additives that are used in protein crystallization they include:
(v) Osmolytes, co-solvents and cosmotropes are compounds that exert their effects at relatively high concentrations, 1 M or more, and include a wide range of molecules such as sucrose, trehalose and other sugars, proline, TMAO, glycine, betaine, taurine, sarcosine and a host of others. The effect of their inclusion in the mother liquor is to stabilize (or destabilize) the native conformation of the protein by altering the interaction of the surface of the protein with water, or by altering the hydration layer and possibly the structured waters.
As an example I cite the seven molecules of glycerol in 1B6G (shown below), a haloalkane dehalogenase from the bacterium, Xanthobacter autotrophicus. Although certain, usually pathogenic, bacteria do have glycosylation pathways, there is no suggestion that this is the case here, as indicated by the nature of the triose and the fact that the crystallization was performed in a glycerol solution:
Subsequently, the crystal was equilibrated for 0.5 h in a solution containing 70% ammonium sulfate and 100 mM MES buffer pH 5.0 and was then soaked for 3 h at room tempera- ture in 10 mM 1-chloropentane, 70% ammonium sulfate and 100 mM citrate buffer pH 5.0. A solution of 30%(w/v) PEG 6000, 20%(v/v) glycerol and 100 mM citrate pH 5.0 was applied as cryoprotectant during data collection.
Haloalkane dehalogenase, 1B6G. The glycerol molecules (grey and red) are shown in space-filling mode. (The yellow and red molecule is sulphate, the green one a chloride ion.)