Control of matrix metalloproteinase catalytic activity

Abstract

As their name implies, MMPs were first described as proteases that degrade extracellular matrix proteins, such as collagens, elastin, proteoglycans, and laminins. However, studies of MMP function in vivo have revealed that these proteinases act on a variety of extracellular protein substrates, often to activate latent forms of effector proteins, such as antimicrobial peptides and cytokines, or to alter protein function, such as shedding of cell-surface proteins. Because their substrates are diverse, MMPs are involved in variety of homeostatic functions, such as bone remodeling, wound healing, and several aspects of immunity. However, MMPs are also involved in a number of pathological processes, such as tumor progression, fibrosis, chronic inflammation, tissue destruction, and more. A key step in regulating MMP proteolysis is the conversion of the zymogen into an active proteinase. Several proMMPs are activated in the secretion pathway by furin proprotein convertases, but for most the activation mechanisms are largely not known. In this review, we discuss both authentic and potential mechanisms of proMMP activation.

Fig. 1. Domain Structure of Mammalian MMPs

Although MMPs are often subdivided into groups based on differences in domain composition, there is little consensus in the field as to how such subdivisions should be assigned. Domain structure alone does not predict function nor substrate preference. However, two clear divisions can be made: 1) MMPs that are secreted or anchored in cis to the cell surface; and 2) MMPs that contain a furin-recognition motif and those that do not. Nine MMPs, including all the membrane anchored enzymes, have a furin-recognition domain; how the others (17 in human; 18 in mice) are activated is the focus this review. The anchored MMPs link to the cell surface via a transmembrane domain (TM; MMP-14, -15, -16, and -24), a GPI linkage (MMP-17 and -25), or an N-terminal signal anchor (SA; MMP-23). As discussed in the text, the so-called “secreted” MMPs may still be confined to the cell surface via interactions with specific adaptor macromolecules.

Fig. 2. Mechanisms of ProMMP Activation

Latency of MMPs (i.e., proMMPs) is maintained by an electrostatic interaction between the free thiol of a conserved cysteine in the prodomain with the HIS-ligated zinc atom in the catalytic pocket. In this state, the prodomain covers the catalytic cleft thereby barring an interaction with a protein substrate. Proteolytic cleavage of the prodomain by furin or other proteases removes the thiol constraint. The thiol-Zn interaction can also be disrupted by non-proteolytic means, and in the lab, this is easily with organomurcials, such as APMA, or SDS. In cells and tissues, proMMPs can anchored to other macromolecules, such as integrins and proteoglycans, and these interactions can lead to allosteric disruption of the thiol-Zn bond. Even though the pro-domain may not be cleaved, the MMP will be active. However, final activation of MMP seems to involve prodomain cleavage, which following an conformational perturbation of the cysteine switch, seems to be controlled by autolysis.

Fig. 3. Control of MMP Activation and Activity by Adaptors

In vitro, proteolysis requires only a proteinase and a substrate, but in vivo, at least one additional component is typically included to augment, if not define, specificity, as well as perhaps catalytic rate. These accessory factors could (and do) come in various flavors: protein or glycosaminoglycans, membrane-associated or extracellular matrix. As suggested by this diagram, a transmembrane accessory factor would simultaneously interact with the proteinase and substrate, bringing both together at an effective concentration. In addition, the activation of some proMMPs, autolytic or otherwise, may be mediated by interaction with these factors, among other potential functions.