Peptide aptamers (PAs) are small, artificially engineered proteins conceptually similar to antibodies . PAs consist of a stable, ideally inert scaffold protein with an inserted constrained peptide moiety. This in effect presents a small peptide surface within the tertiary structure of the scaffold which serves as the binding site for a target protein. In contrast to most of the more than 40 non-antibody scaffolds described to date , PAs are usually isolated by yeast-two hybrid screening of large libraries of PAs that contain random peptide inserts against a bait protein of interest. Selection of PAs in eukaryotic cells in vivo may allow the identification of interactors that are more easily transferable to mammalian cells than interactors identified using in vitro techniques such as phage-display. PA technology is well established, with PAs showing biological activity against a wide variety of proteins from different organisms, including the human and D. melanogaster Cdk2 proteins [1, 3], the E. coli thymidylate synthase (ThyA) protein , the E6 and E7 proteins from human papilloma virus (HPV) [5, 6], the human EGF receptor , and the transcription factors Stat3  and the BCL-6 . Importantly, some PAs have also been found to block functions of their target proteins in vivo, such as human Cdk2 , D. melanogaster Cdk1 and 2 , E2F , p53 , Stat3 , Nr-13 , and BCL-6 .
Membrane-type 1 Matrix Metalloproteinase (MT1-MMP, also known as MMP-14), is a member of the large MMP family of enzymes. MT1-MMP plays a major role in the dynamic remodelling of the extra-cellular matrix (ECM) and has been reported to directly degrade a broad spectrum of ECM proteins, including collagen types I, II, and III, fibronectin, laminin 1, laminin 5, fibrin, and aggrecan [14, 15]. MT1-MMP has also been reported to activate proMMP-2 and proMMP-13 [16, 17], thereby indirectly increasing its proteolytic repertoire on or near the cell surface. The protease also plays a role in the processing of a growing number of membrane proteins, including, for example, CD44 , transglutaminase , the integrin αV chain  or syndecan 1  thus modulating cell signalling and the cellular functions mediated by these molecules.
MT1-MMP has been implicated in a wide spectrum of physiological and pathological cellular functions [22, 23]. MT1-MMP expression, well documented in many tumours, has been correlated with key in vitro and in vivo processes of tumour progression including angiogenesis , cell migration and invasion , cell growth  and metastatic spread [27, 28]. Inhibition or silencing of the protease has been found to significantly reduce the invasive phenotype of tumour cells implicating a leading role for MT1-MMP in such processes [25, 29].
MT1-MMP is a type I transmembrane protein with a very short intracellular domain (ICD) of just 21 amino acids. The MT1-MMP ICD has been reported to be required for cell migration and invasion [30–33] as well as tumour growth . The identification of proteins interacting with the MT1-MMP ICD, such as MTCBP-1 , and glCqR  have also helped in defining new localisations and cellular functions for this protease. The MT1-MMP ICD has also been implicated in the internalisation  and the recycling of the protease to the cell surface . Consistent with this, MT1-MMP ICD has been reported to interact with the μ2 subunit of the AP-2 complex  as well as with caveolin-1 
To date, crucial information on the cellular function of the intracellular domain of the protease has been obtained following exogenous expression of mutant MT1-MMP ICD constructs [31, 32, 39, 38, 37, 40, 41, 33] or constructs with a partially or completely deleted ICD [30, 42, 26, 43, 40, 34]. In order to assess the role of the MT1-MMP ICD without using exogenously truncated or mutated forms of the protease, we decided to make use of PA technology.
In this study, we identify and characterize a PA, named swiggle, which interacts with the 21 amino acid ICD of MT1-MMP. Expression of swiggle in human cells was found to stimulate MT1-MMP mediated cell migration. Detailed analysis of the phenotypic effect of swiggle revealed that the PA inhibits internalization of MT1-MMP resulting in the accumulation of the protease at the cell surface. Our data indicate that swiggle interacts with the LLY573 motif in the MT1-MMP ICD and competes with the μ2 subunit of the AP-2 complex in cells, thereby inhibiting the endocytosis of MT1-MMP.