|Description||Members of the ADAM (A Disintegrin And Metalloprotease) protein family uniquely exhibit both proteolytic and adhesive properties. Specifically, ADAMs catalyze the conversion of cell-surface proteins to soluble, biologically active derivatives through a process known as ectodomain shedding. Ectodomain shedding coordinates normal physiological processes. Aberrant ADAM activity contributes to pathological states, such as chronic inflammation. Understanding how ADAM ectodomain shedding activity is governed may provide new avenues for therapeutic intervention of ADAM-mediated shedding pathologies.
While ectodomain shedding is the hallmark feature of the ADAMs, thirteen of the forty ADAMs identified among various species are catalytically inactive. Noncatalytic ADAMs lack one or more consensus elements (HExxHxxGxxH) within the active site of the metalloprotease domain. Despite lacking the hallmark catalytic activity, noncatalytic ADAMs exhibit function(s) associated with other nonenzymatic domains (e.g. integrin recognition of the disintegrin domain). Disruption/mutation of noncatalytic ADAMs has been associated with perturbation of biological events.
My overall hypothesis is that noncatalytic ADAMs regulate the activity of catalytically active ADAMs by competing for substrates and/or receptors when expressed within the same cellular niche. To begin testing this proposed competitive binding regulatory mechanism, I used noncatalytic human ADAM7 and catalytically active human ADAM28 as a model ADAM pair. Preliminary, unpublished data from our lab demonstrated expression of ADAM7 mRNA in multiple immune cell lines established to express ADAM28 at the protein level. For determination of ADAM7 expression patterns, monoclonal antibodies against ADAM7 were produced by our lab. However, the antibodies failed to exhibit reactivity against exogenous, full-length ADAM7.
Based upon preliminary phylogenetic analysis and genomic location, it is likely that ADAM7 arose from gene duplication of ADAM28, which would allow a genetic copy of the molecular specificity required for regulation (e.g. integrin binding) with eventual silencing of catalytic activity. We predicted that the gross structural integrity of the metalloprotease may be uniquely conserved between ADAM7 and ADAM28. Restoration of the active site glutamate residue of the ADAM7 metalloprotease domain bestowed catalytic activity to ADAM7 in a manner that reflected specificity of ADAM28-mediated catalysis. This is the first demonstration, to our knowledge, of 'awakening' a noncatalytic enzyme through a single point mutation. This discovery provides an initial functional link between ADAM7 and ADAM28 and lends credence to the hypothesis that ADAM7 may regulate ADAM28 through competitive binding. These findings have a broader impact, as 92 of the 570 collective human proteases are noncatalytic.||