Undergraduate Research Journal

Molecular Analysis of the Interaction Between Staphylococcus aureus Protein Sbi and Immune System Protein C3d
Wilson Rodriguez
Introduction
The complement system is part of our innate immune
system and delivers potent responses to pathogenic
infection and disease. Complement component 3 (C3)
serves as the central protein of the complement system
and can be activated by the classical, lectin, and alternative
pathways. Cleavage of C3 results in fragments C3a and
C3b, which can participate in amplification of immune
response through the formation of convertases, as well as
proinflamatory responses, chemotaxis, and the formation
of the membrane attack complex (MAC). From these
fragments, C3b tags pathogens, through the formation of a
thioester bond, for subsequent recognition and elimination
by macrophages. C3a interacts with C3aR on the surfaces of
leukocytes and induces cell mobilization and chemotaxis.
MAC is responsible for directly killing pathogens by cell
lysis. 9 Simultaneous interactions with factors H and I lead
to cleavage of C3b, yielding iC3b (inactive C3b) and C3d,
with both fragments containing the thioeter bond moiety.
C3d interacts with Complement Receptor 2 (CR2) and
antigen-antibody complexes to facilitate B- and T- cell
immune response. 7 Through the C3d fragment and the
formation of the B cell receptor-coreceptor complex, the
complement system acts as a link between innate and
adaptive immunities by augmenting antibody responses.
the C3d-CR2 complex from forming and thus blocking C3d
from interacting with other branches of the host’s immune
system such as the B and T cells. It is also hypothesized
that Sbi interacts with residues involved in thioester
bond formation between C3d and bacterial surfaces, thus
inhibiting binding to pathogenic cell surfaces and retarding
the formation of convertases in the classical pathway. 3
In this study we analyze the effects of mutant variants,
which are predictive of protein binding and stability.
Through the generation of mutant variants and the
calculation of electrostatic potentials, we can elucidate
residues critical to the binding mechanism. 1 With these
computational mutagenesis results we can model and
design therapeutics tailored to enhance or inhibit specific
protein-protein interactions.
Although our immune responses are able to combat most
infections, most bacterial pathogens have evolved and
adapted methods for evading the human complement
immune system. S. aureus in particular has developed a
variety of immuno-regulatory proteins for the goal of
immune evasion and does so by regulating the innate and
adaptive immune systems. S. aureus also has the ability to
control host regulatory proteins, thus facilitating bacterial
function within the host. 7 One recently identified protein
responsible for a variety of immune evasive functions
is S. aureus binder of immunoglobulin G (Sbi). Sbi
affects the adaptive immune system by binding to host
immunoglobulin G3, but Sbi can also affect the alternative
pathway of complement by interacting with C3d. Several
experiments revealed that Sbi binds to C3d in a one-toone
ratio but can bind to C3d at two locations: Complex 1 and
Complex 23. This mode of binding allows Sbi to potently
counter the complement activation. In Complex 1, Sbi inhibits
Figure 1. Complement component 3 fragment d (C3d) shown
in cyan ribbon representation, Sbi shown in magenta at two
binding sites (note simultaneous C3d-Sbi of complex 1 and 2
interaction). PDB code: 2WY8 and 2WY7, respectively. 4 (A) C3d-
Sbi interaction at concave side responsible for inhibiting C3d-
CR2 interaction. (B) Possible C3d-Sbi interaction responsible of
blocking thioester domain which binds to pathogenic cell surface.
Methods
Because electrostatics plays a critical role in protein-protein
interactions, we explored the specific role of electrostatics
in C3d-Sbi interaction. In order to elucidate the binding and
function mechanism of C3d-Sbi complex, the computational
framework Analysis of Electrostatic Similarities Of Proteins
(AESOP) was employed. 5-9 The AESOP framework is a
2 4 U C R U n d e r g r a d u a t e R e s e a r c h J o u r n a l