Abstract
The hemopexin-like domain of membrane-type matrix metalloproteinase-1 (MT1-MMP) enables MT1-MMP to form oligomers that facilitate the activation of pro-matrix metalloproteinase-2 (pro-MMP-2) at the cell surface. To investigate the role of the MT1-MMP hemopexin domain in the trafficking of MT1-MMP to the cell surface we have examined the activity of two MT1-MT4-MMP chimaeras in which the hemopexin domain of MT1-MMP has been replaced with that of human or mouse MT4-MMP. We show that MT1-MMP bearing the hemopexin domain of MT4-MMP was incapable of activating pro-MMP-2 or degrading gelatin in cell based assays. Furthermore, cell surface biotinylation and indirect immunofluorescence show that transiently expressed MT1-MT4-MMP chimaeras failed to reach the plasma membrane and were retained in the endoplasmic reticulum. Functional activity could be restored by replacing the MT4-MMP hemopexin domain with the wild-type MT1-MMP hemopexin domain. Subsequent analysis with an antibody specifically recognising the propeptide of MT1-MMP revealed that the propeptides of the MT1-MT4-MMP chimaeras failed to undergo proper processing. It has previously been suggested that the hemopexin domain of MT4-MMP could exert a regulatory mechanism that prevents MT4-MMP from activating pro-MMP-2. In this report, we demonstrate unambiguously that MT1-MT4-MMP chimaeras do not undergo normal trafficking and are not correctly processed to their fully active forms and, as a consequence, they are unable to activate pro-MMP-2 at the cell surface.