Dimerization of lactase-phlorizin hydrolase occurs in the endoplasmic reticulum, involves the putative membrane spanning domain and is required for an efficient transport of the enzyme to the cell surface.

Analysis of the quaternary structure of human intestinal lactase-phlorizin hydrolase (LPH) by chemical cross-linking and sucrose-gradient centrifugation reveals that the brush border form of LPH (LPH beta; 160-kDa) is a homodimeric molecule. Dimerization ensures in the ER when LPH is still exclusively found as an uncleaved mannoserich precursor (pro-LPHb; 215-kDa). This is supported by the following observations. (i) Biosynthetically labeled intestinal biopsy specimens as well as transfected COS-1 cells expressing pro-LPH contain monomeric and dimeric forms of pro-LPHb; the complex glycosylated pro-LPH (pro-LPHc; 230-kDa) as well as the cleaved mature LPH beta species in intestinal biopsy samples are discerned exclusively as dimers. (ii) Dimeric forms of pro-LPHh could be also detected when cells were biosynthetically labeled at 15 degrees C, at which temperature the egress of pro-LPH from the ER is blocked. Dimerization is essential for the transport competence of pro-LPH and is strongly associated with the presence of an intact transmembrane domain. Mutant pro-LPH-mact lacking the complete transmembrane domain persists as a monomeric, mannose-rich and transport-incompetent molecule that is not secreted into the exterior milieu, accumulates most likely in the ER and is ultimately degraded. Further, deletion of the cytoplasmic tail in the pro-LPH-ct mutant leads to marked reduction in the proportion of dimeric as well as complex glycosylated pro-LPH-ct. Finally, dimerization is linked to the acquisition of LPH to its biological function, since only dimers of wild type pro-LPH or pro-LPH-ct are enzymatically active, while their monomeric counterparts as well as pro-LPH-mact are not.