Protein bands visible on unstained SDS-PAGE - (Oct/22/2014 )

While studying a mix of unknown proteins from a secretory product of insect origin I noticed an unusual effect: some of the proteins bands, particularly towards the bottom of the gel, become visible immediately after fixation (in acetic acid-ethanol fix), i.e. prior to Coomassie staining. These bands appear whitish, milky, opalescent and are perfectly visible especially when the gel is being held against a dark background. They are slightly offset compared to Coomassie-stained bands and are more diffuse. This suggests that this is not protein itself, but perhaps something else associated with a protein. I thought this might be sugars and did staining for glycolized proteins, but the results didn't match well the pattern of milky bands. I think their opalescent appearance is due to some kind of precipitation within the gel. I never saw it in other cases, including negative controls and ladders on the same gels. These bands develop after gel fixation after both glycine- and tricine-based SDS-PAGE and seem to be a feature of the material I study. Any ideas what it could be? Needless to say, I would be extremely grateful for any ideas.

Proteins precipitating within the gel is not uncommon, especially a protein at high concentration in your sample.  I know in mammalian muscle samples we frequently see myosin heavy chain precipitate in the gel.