Hi Veronique, if you did not sonicate the sample or shear it before centrifugation I would guess that you have mitochondria, nuclei, DNA/RNA aggregates, insoluble proteins and lipid rafts in your TX insoluble pellet.

A fraction of the plasma membrane is also insoluble in Triton (detergent-insoluble membranes), these are rich in sphingolipids and cholesterol and correspond at least partially to "rafts". Because rafts (~ 1e6 per cell) are thicker than the phospholipid-rich sea in which they swim, they also attract a different set of transmembrane proteins with longer helices.

Thanks a lot both it is really helpful.

Sorry, mistake click