The nonideal coiled coil of M protein and its multifarious functions in pathogenesis

Abstract

The M protein is a major virulence factor of Streptococcus pyogenes (group A Streptococcus, GAS). This gram-positive bacterial pathogen is responsible for mild infections, such as pharyngitis, and severe invasive disease, like streptococcal toxic shock syndrome. M protein contributes to GAS virulence in multifarious ways, including blocking deposition of antibodies and complement, helping formation of microcolonies, neutralizing antimicrobial peptides, and triggering a proinflammatory and procoagulatory state. These functions are specified by interactions between M protein and many host components, especially C4BP and fibrinogen. The former interaction is conserved among many antigenically variant M protein types but occurs in a strikingly sequence-independent manner, and the latter is associated in the M1 protein type with severe invasive disease. Remarkably for a protein of such diverse interactions, the M protein has a relatively simple but nonideal α-helical coiled coil sequence. This sequence nonideality is a crucial feature of M protein. Nonideal residues give rise to specific irregularities in its coiled-coil structure, which are essential for interactions with fibrinogen and establishment of a proinflammatory state. In addition, these structural irregularities are reminiscent of those in myosin and tropomyosin, which are targets for crossreactive antibodies in patients suffering from autoimmune sequelae of GAS infection.

Fig. 12.1

Regions of M protein. Top, the M1 protein type is used as an example to delineate regions. The signal sequence (SS) is removed proteolytically, and the C-terminal “LPXTG” motif is cleaved and covalently attached to the peptidoglycan. Mature M1 is constituted by the A-region, which contains the 50-residue long hypervariable region (HVR); the B-repeats, which consist of ~2.2 imperfect repeats of a 28-residue sequence; the S-region, which is unique to M1; the C-repeats, which consist of 3 imperfect repeats of a 42-residue sequence; and the D-region. Bottom, most of mature M1 has a high propensity for forming an α-helical coiled coil, with a slight break at the end of the B-repeats

Fig. 12.2

Crystal structure of M protein. (a) Knobs-into-hole packing of residues in the a and d positions, left and middle, respectively, in a dimeric, parallel α-helical coiled coil. Both a and d positions in this example are occupied by Leu, and examples of “knobs” and “holes” are labelled. Right, knobs-into-hole packing of an Asp at the a position with a Leu in the d position in a dimeric, anti-parallel α-helical coiled coil. This and other molecular figures were generated with PyMol (http://www.pymol.org). (b) Structure of the M1^AB α-helical coiled-coil dimer. The HVR is in red (with its ~15 initial residues disordered and shown in an arbitrary position as dashed lines), the rest of the A-region in blue, and the B-repeats in green. In yellow, residues that form the Ala-stagger and cause unwinding due to large positively charged residues at a positions. This region is boxed and expanded below. (c) The B-repeats of the M1^AB dimer splay apart and engage in an anti-parallel α-helical coiled-coil with the B-repeats of a neighbouring molecule. This anti-parallel interaction between two neighbouring M1^AB dimers is shown, with one α-helix of each dimer omitted for clarity

Fig. 12.3

Functional interactions of M protein. Bacterial surface-bound M protein contributes to (a) GAS aggregation through homotypic interactions, and (b) evasion of phagocytosis through recruitment of fibrinogen or C4BP to the bacterial surface. Bound fibrinogen is also responsible for recruiting plasmin to the GAS surface, which is associated with a transition from localized to invasive infection. (c) M protein released by neutrophil proteases from the bacterial surface interacts with fibrinogen, and M-fibrinogen complexes activate neutrophils via β₂ integrins along with IgGs that bind to M protein and interact with FcγRII. Activated neutrophils release the vasodilator heparin binding protein (HBP). (d) M-fibrinogen complexes also activate platelets. This occurs through interaction of these complexes with the integrin GPIIb/IIIa along with IgGs that bind to M protein and interact with FcγRII. Activated platelets in turn lead to further activation of neutrophils and monocytes. (e) M protein synergizes in a TLR2-dependent manner with HBP to activate monocytes, which then secrete proinflammatory cytokines and upregulate the procoagulatory protein tissue factor. (f) M protein is also responsible for neutralizing the antimicrobial effects of cathelicidins in neutrophil extracellular traps (NET)