The streptococcal collagen-like proteins Scl1 and Scl2 are prokaryotic members of a large protein family with domains containing the repeating amino acid sequence (Gly-Xaa-Yaa) n that form a collagen-like triple-helical structure. Here, we test the hypothesis that Scl variant might interact with mammalian collagen-binding integrins. We show that the recombinant Scl protein p176 promotes adhesion and spreading of human lung fibroblast cells through an ␣ 2 ␤ 1 integrin-mediated interaction as shown in cell adhesion inhibition assays using anti-␣ 2 ␤ 1 and anti-␤ 1 integrins monoclonal antibodies. Accordingly, C2C12 cells stably expressing ␣ 2 ␤ 1 integrin as the only collagen-binding integrin show productive cell adhesion activities on p176 that can be blocked by an anti-␣ 2 ␤ 1 integrin antibody. In addition, p176 promotes tyrosine phosphorylation of p125 FAK of C2C12 cells expressing ␣ 2 ␤ 1 integrin, whereas parental C2C12 cells do not. Furthermore, adhesion of human lung fibroblast cells to p176 induces phosphorylation of p125 FAK , p130 CAS , and p68 Paxillin proteins. In a domain swapping experiment, we show that integrin binds to the collagenous domain of the Scl protein. Moreover, the recombinant inserted domain of the ␣ 2 integrin interacts with p176 with a relatively high affinity (K D ‫؍‬ 17 nM). Attempts to identify the integrin sites in p176 suggest that more than one site may be involved. These studies, for the first time, suggest that the collagen-like proteins of prokaryotes retained not only structural but also functional characteristics of their eukaryotic counterparts.The collagens are a family of extracellular matrix (ECM) 1 proteins that provide structural support to all multicellular animals (1). The repeating sequence Gly-Xaa-Yaa (GXY), where X is often proline and Y is often hydroxyproline, is a unique feature of the collagen polypeptides (2-4). Long tracks of repeated GXY sequences fold into left-handed polyproline type II-like chains, and three such chains cooperatively twist around a central axis to form a right-handed rope-like superhelix (2, 5-7), considered a defining feature of collagens. The collagens also contain C-and N-terminal noncollagenous domains, which often are proteolyzed during secretion from the cells (7-9). Mature collagen molecules are deposited in the ECM in the form of fibers, networks, and beaded filaments (8). One group of mammalian proteins that fulfill rudimentary host defense functions such as the complement factor C1q (9) and several mammalian lectins (10) also have collagenous domains, but they are not conventional collagens. These proteins form characteristic lollipop-like structures, where the collagenous domains form the stalks, and globular heads correspond to the noncollagenous regions. Collagen-like molecules also have been found in lower eukaryotes such as mussels, worms, and sponges (11), and collagen-like sequences have been identified from analyses of the genomes of prokaryotes (12-15).The streptococcal collagen-like proteins Scl1 and Scl2 (also know...

SummarySeveral bacterial genera express proteins that contain collagen-like regions, which are associated with variable (V) non-collagenous regions. The streptococcal collagen-like proteins, Scl1 and Scl2, of group A Streptococcus (GAS) are members of this 'prokaryotic collagen' family, and they too contain an amino-terminal non-collagenous V region of unknown function. Here, we use recombinant rScl constructs, derived from several Scl1 and Scl2 variants, and affinity chromatography to identify Scl ligands present in human plasma. First, we show that Scl1, but not Scl2, proteins from different GAS serotypes bind the same ligand identified as apolipoprotein B (ApoB100), which is a major component of the low-density lipoprotein (LDL). Scl1 binding to purified ApoB100 and LDL is specific and concentration-dependent. Furthermore, the noncollagenous V region of the Scl1 protein is responsible for LDL/ApoB100 binding because only those rScls, constructed by domain swapping, which contain the V region from Scl1 proteins, were able to bind to ApoB100 and LDL ligands, and this binding was inhibited by antibodies directed against the Scl1-V region. Electron microscopy images of Scl1-LDL complexes showed that the globular V domain of Scl1 interacted with spherical particles of LDL. Importantly, live M28-type GAS cells absorbed plasma LDL on the cell surface and this binding depended on the surface expression of the Scl1.28, but not Scl2.28, protein. Phylogenetic analysis showed that the noncollagenous globular domains of Scl1 and Scl2 evolved independently to form separate lineages, which differ in amino acid sequence, and these differences may account for the variations in binding patterns of Scl1 and Scl2 proteins. Present studies provide insight into the structure-function relationship of the Scl proteins and also underline the importance of lipoprotein binding by GAS.