Proteasomes are usually localized to the cytoplasm but are also present in the nucleus and ER. Houde et al. (2003) investigated the distribution of proteasomes in J774 macrophages and observed them on phagosomes. The proteasomes present on phagosomes may not come directly from ER, but instead assemble on phagosomes to play a function at a precise point during phagolysosomal biogenesis. Houde et al. also observed polyubiquitinated proteins on the cytoplasmic side of phagosomes and highlighted the link between the ubiquitination process and proteasomal degradation; translocated peptides are ubiquitinated and processed by the proteasome complex assembled on the cytoplasmic side of phagosomes, leading to the generation of MHC class I binding peptides.