cd-mpr: the cd-mpr is much smaller than the ci-mpr – it is only ~46 kda. its n-terminal extracytoplasmic domain contains only 1 p-type carbohydrate recognition domain. the cd-mpr exists mainly as a dimer in the membrane. however monomeric and tetrameric forms are also thought to exist as well. the equilibrium between these different oligomers is affected by ph, temperature and presence of mannose 6-phosphate residues. each monomer forms a 9 stranded ß-barrel which can bind to a single mannose 6-phosphate residue.