A unique C-terminal domain allows retention of matrix metalloproteinase-27 in the endoplasmic reticulum

Abstract

We report that, unlike other matrix metalloproteinases (MMPs), MMP-27 is retained in the endoplasmic reticulum due to its specific C-terminal extension (CTE). Upon CTE removal, MMP-27 accesses the intermediate compartment and Golgi and is constitutively secreted. Conversely, CTE addition to a secreted MMP (MMP-10) causes ER retention and blocks secretion. We further show that MMP-27 is not an integral membrane protein. Indeed, the CTE is not a transmembrane domain and MMP-27 partitions in the aqueous phase after Triton X-114 extraction. Matrix metalloproteinase-27 (MMP-27) is poorly characterized. Sequence comparison suggests that a C-terminal extension (CTE) includes a potential transmembrane domain as in some membrane-type (MT)-MMPs. Having noticed that MMP-27 was barely secreted, we investigated its subcellular localization and addressed CTE contribution for MMP-27 retention. Intracellular MMP-27 was sensitive to endoglycosidase H. Subcellular fractionation and confocal microscopy evidenced retention of endogenous MMP-27 or recombinant rMMP-27 in the endoplasmic reticulum (ER) with locked exit across the intermediate compartment (ERGIC). Conversely, truncated rMMP-27 without CTE accessed downstream secretory compartments (ERGIC and Golgi) and was constitutively secreted. CTE addition to rMMP-10 (a secreted MMP) caused ER retention and blocked secretion. Addition of a PKA target sequence to the cytosolic C-terminus of transmembrane MT1-MMP/MMP-14 led to effective phosphorylation upon forskolin stimulation, but not for MMP-27, excluding transmembrane anchorage. Moreover, MMP-27 was protected from digestion by proteinase K. Finally, MT1-MMP/MMP-14 but neither endogenous nor recombinant MMP-27 partitioned in the detergent phase after Triton X-114 extraction, indicating that MMP-27 is not an integral membrane protein. In conclusion, MMP-27 is efficiently retained within the ER due to its unique CTE, which does not lead to stable membrane insertion. This could represent a novel ER retention system. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.